I wanted to build a large area of pistons facing down and wanted to use FastBlockPlacement and rotate the piston with flexibleBlockPlacementRotation. For one Block this works fine (I do have carpet + carpet extra on the server) and the piston rotates correctly. But FastBlockPlacement doesn't work. After one piston is placed nothing happenes when keeping the right mouse button presssed. If i disable flexibleBlockPlacement, FastBlockPlacement works again, but then i do get incorrectly rotated pistons.

How do I get to use both tweaks at the same time in particular for pistons?

Which placement restriction mode are you using with FastBlockPlacement? Does the issue happen with many/all modes? And do you still have the rememberFlexibleFromClick option enabled? You don't have grid placement or placement limit or some of the other restriction tweaks enabled as well?

Which MC version are you in? There was a bug fix to the flexible placement stuff in one of the recent-ish MC version ports, can't remember off the top of my head if it was in 1.19.x or 1.20.x. Although I think that was mainly for incorrect rotation with offset + rotation or offset + accurate, so that likely wouldn't affect this case.

I'm in 1.20.1 and I have rememberFlexibleFromClick enabled. I don't have any special grid placement or restriction tweaks enabled. I got it to work for my use case with the Layer Restriction mode. With layer it works like it should.
I haven't gotten it to work with Face though. Thats what I initially used and therefore made the issue here.
I haven't tested the other restriction modes, but Layer works and Face doesn't.