Why this faces changes position when making it a group?

Can you share the file? Are all the drivers of your gpu up to date?

As @francisquitof implied, it is hard to be sure what is going on without examining the .skp file. If you upload it here, folks will check whether they see the same issue and can find a cause.

Edit: assuming this involves the same model as in your other post, it is likely that these small mismatches between the adjacent parts of the couch back are to blame.

proxy sofas.skp (14.1 MB)
here it is

sorry for the delay. I didnt see this one