Steps to reproduce:
1. place down a wooden pressure_plate
2. run

summon area_effect_cloud ~ ~ ~ {Duration:99}

while standing on the pressure_plate.
3. The pressure_plate is now powered by the player. if you step off, its still powered!
4. if you kill the aec (or just wait for it to despawn), the pressure_plate turns off.

If the pressure_plate doesn't get powered by another entity, the aec won't affect it.

This does not just happen in custom maps (which makes it more complicated to implement certain mechanics), I even managed to reproduce it in "vanilla survival" with the "dragon breath" aec.