FilmPack requires the use of Rosetta mode to run on Mac Studio. In both the standalone and plug-in, FP looks fine. But the saved results appear to show only part of the image – magnified.
What are the versions of FP and macOS we’re talking about?
System Profiler of macOS Monterey 12.6.8 recognises FP 6.13 to be “Universal”.
FP 6.13 should therefore be able run without Rosetta.
MacOS 13.5 Ventura, FP 13.0
Additional tests in MacOS Ventura (13.5).
FilmPack 6.13 runs correctly as a standalone and as a plug-in (tested in PhotoLine) without Rosetta.
The FilmPack 6.13 plug-in does not run correctly in Affinity Photo 2.1.1.
Things look good with FP 6.12 and 6.13 on macOS Sonoma Beta…except that I cannot export because my trial period has expired.
Affinity is kind of picky about how the plugins have been installed. Affinity support (documents) might be able to help.
If I were in your shoes, I’d try to remove FilmPack completely and a new install - after verifying that Affinity is correctly set up in regard to the plugins.