Steps to reproduce:
Put down piston.
Put redstone block next to it.
Cast EMP.

Did not do this in magic 8.3.3

I think this is a long-standing bug with piston undo that was probably exacerbated by EMP now toggling redstone blocks... but I think it was always there, if you had thrown a button or level into the mix it probably would've happened anyway.

I'd like to get it fixed, pistons are weird, I'll see what I can do.

In the meantime if you want to put it back the way it was (maybe slightly safer) you can do

/mconfig configure spell emp parameters.destructible lever,buttons