Yes, it doesn’t make sense. PLA and PETG are different plastic but melting points are very similar and from the point of view of the printer, they are pretty much the same.
It’s like your car only ran on ron91 but not ron95.
I can only make wild guesses. Like, your plate somehow became solft with PETG but hard with PLA or a phantom object got in the way of the nozzle when it do the tapping.
We need more info, as much as possible,like, something small and obvious to us but might not to you and vice versa.
I think record videos is the best way, for both cases: normal tapping when doing PLA printing and fail tapping when doing PETG printing.
Read this thread for more info about the auto leveling process Bed Leveling Process
On another guess, it could be the connector of the piezo sensors is coming lose a bit just enough for it not to register the signal from a tap. This guy https://www.youtube.com/watch?v=Fb3LNc1WQnA had that problem and fixed it by taping the sensor wires to the poop chute.