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I am happy to report that the issue has been resolved—by accident, that is. I am sharing this in the hope that this information may help someone else.

Short version:
Removing the battery power for a while must have flushed some gremlins out of the memory of the AK, and both the front and rear TPM sensors work as expected now.

Long version:
OK, so this happened while I was away on holiday. The battery wasn't hooked up to the Kymco factory trickle charger I received with the scooter while I was away. Additionally, I hadn't ridden for about two weeks before the holidays, so after about a month of not using it, the battery went flat. Not the "it would not crank" kind of flat, but the "the dash didn't come up either" kind of flat! I measured 5.5V through the external charge port connector. I thought the battery was toast. Regardless, I plugged in the trickle charger, and after about 30 minutes or so, the voltage climbed up to 10.1V, so then I hooked it up to a dedicated charger. Once the battery was fully recharged, I took the scooter for a ride. On my way out, the tires felt "soft", so I stopped at a gas station half a mile from my home and topped up the tires. Later it turned out I had over-inflated them a bit due to manometer error at the gas station (Front: 2.55 Bars, Rear: 2.9 Bars, instead of 2.4/2.7). Later, while riding and toggling through the menu just to see my average fuel consumption, I noticed there was a pressure reading from both sensors!!!

Maybe the removal of the battery power, combined with the injection of some more air, worked, but it did.

I had already considered removing one of the battery terminals and leaving it overnight, hoping to find a solution to the issue that way. However, I had already made an appointment with the local Kymco dealer to have it serviced. Plus, it seemed too old-school of a solution, and I wasn't at all confident it would do any good. I was also reluctant to dismantle half the scooter just to access the battery.

I have the same thing on my own and can confirm a friend's AK 550 also has this.
Both scoots have around 30K km on the clock and 2017 model.

These vibrations you describe come around 50km/h +/- 5km/h and they resurface at around 100km/h but in a somewhat lesser way?! (Or less pronounced because of noise/wind speed)
New tires, correct tire pressure, new CVT belt changed nothing.
Final drive belt still from factory but tension have been checked by dealer, still no improvement.
Brake discs and pads ok on front, rear disc has some wear (perhaps parking brake induced?), pads move freely. Fluids bled and replaced.

I am thinking of checking wear on rollers, the part of the variator housing the rollers, variator sleeve/boss and/or replace.
Friend's AK has a Malossi variator fitted but these harmonic vibrations where there with the factory variator and also present after the Malossi was fitted.

So far we could not find the source of the problem.
I am trying to think it is the nature of the beast, but I wish it would run smoother.
Let us know if you find anything.

I woulda thought the sensor would be powered by a piezo electric source and the centrifugal force of the wheel

What would happen to after sales then?
No, that would be too convenient for the owners of the vehicles.

I am reaching out hoping someone can help me as I am at my wits end.

I have just replaced the tires and also put in 2 brand new TPM sensors on my 2017 AK 550 scooter.

I have followed the instructions in the service manual, page 2-35 (Learn Code Activation Procedure) on how to program the new sensors on the scooter.
I could enter TPM learning mode by pressing and holding the ^ button then turning the ignition key on (dashboard switched to "m" position beforehand).
Then I have released some pressure first from the front tire, then the rear as instructed, and seemingly both sensors registered, I saw 2.25 Bar for both front and rear tires.
However, after turning the ignition key off and then back on again all I see now is 0.00 bar for both front and rear tires.
Furthermore, I cannot enter TPM sensor programming mode any longer...?!

What am I missing? What am I doing wrong? And more importantly what can I do to remedy the situation?
Is there a way to reset the sensors or the receiver that communicates with the sensors? Is there a special button sequence to somehow restart the whole code learning procedure?

I have already reset the scooter back to factory default setting (by pressing and holding both o and ? buttons while turning on the ignition key) but it didn't help, I still see the 0.00 Bar for both front and rear tires.

The dealership I bought the sensors from was fully booked for a month and a half in advance so they could not fit the sensors for me.
They sold me the sensors but now they are not willing to help me as I have fitted the sensors myself. Go figure?!

Please if anyone has any ideas please share.
Thanks!

The beamer looks nice but the Yamaha  T max aint bad either.
To stay faithful to the brand the K50 from Kymco looks very promising too...

-on a 10amp scale, the negative wire & negative battery post draws 5.4 amps key on.

Is this a continuous current draw or only present while the fuel pump runs for a few seconds when you turn the ignition key to on position?
If this current draw is continuous with the ignition key on, then that's what causes your battery to collapse after a short while.
That's over 60W of power drain. Which also has to create considerable amount of heat somewhere.

I am trying to help. Please keep posting your findings.

I've got power pure s both front and rear. They are fantastic. Can't even compare to stock doughnuts. I can only recommend them.