You mean with Rosetta you can edit .xib files? I haven't tried with .xib files, but I can edit a .storyboard file with Rosetta which does not open without Rosetta (shows "Internal Error"). EDIT: Sorry I had this the other way around. The .storyboard edits without Rosetta, and does not open with Rosetta. It will only compile with Rosetta.

Quick question for you guys, what version of macOS are you running? I realized this has started happening after I updated to 11.2.3. Might be a coincidence though. I just tested mine and it's working. I'm on macOS 11.2.2. I'm running Xcode 12.3 with Rosetta and 12.4 without Rosetta.