We have a new geberit concealed tank and duravit wall hung toilet.
Occasionally, after flushing, the tank continues to pour water into the bowl so that the tank does not fill.

I assume the flapper has not closed?

The problem with these tanks is getting access to the parts - we can't even see the flapper.

Anybody seen this before - any ideas?

It seems to occur more frequently if the shower is on, or a faucet is running - but its also occurs when neither of these are on.

The original plumber who did the install (as part of remodel) didn't have a clue and is not being helpful