By testing with my alt using PojavLauncher, the pressure plate sometimes teleports you to just under the floor level Y=84 (i.e. Y=83.xxx) due to some slight connection delays, and it let your feet sink into the ground & getting repeatedly teleported up and down between like 83.000 and 83.999. But the speedometer remains zero since server side still consider your actual location to be Y=84.000.
For this case, you may still move yourself to the center of the elevator to get rid of the teleportation cycle & get you back to Y=84.000, and then get through the pressure plate again after it gets released.
So, was it the same case that you encountered last weekend? 🤔
