Thanks for reporting this. After looking into it, unfortunately it's not reproducible here.
During this Apple Silicon transition, particularly with it occurring "midstream" with Illustrator 2021 (i.e. 25.0 was Intel-only but 25.3 was a Universal Binary), we noticed a variety of issues not only with plugins but also with Illustrator itself when running on a version of macOS prior to 11.0. My suspicion is that, as Illustrator itself is now built with the macOS 11.1 SDK (though technically targeted back to macOS 10.15), maximum compatibility occurs with 11.0 onwards and occasional bugs are more likely with macOS 10.15.
So for anyone running Illustrator 25.3 or 26.x on macOS 10.15, my recommendation is to upgrade to macOS 11.0+ and see if the issues are resolved. In virtually every case we have seen resolution.
The same goes for the "inverse" of running Illustrator 25.0 on macOS 11.0+ with a plugin binary built against the AI25 library - getting these folks to either stay back on macOS 10.15 or to update to Illustrator 25.3 and get a plugin binary built against the AI25.3 library.
During this Apple Silicon transition, particularly with it occurring "midstream" with Illustrator 2021 (i.e. 25.0 was Intel-only but 25.3 was a Universal Binary), we noticed a variety of issues not only with plugins but also with Illustrator itself when running on a version of macOS prior to 11.0. My suspicion is that, as Illustrator itself is now built with the macOS 11.1 SDK (though technically targeted back to macOS 10.15), maximum compatibility occurs with 11.0 onwards and occasional bugs are more likely with macOS 10.15.
So for anyone running Illustrator 25.3 or 26.x on macOS 10.15, my recommendation is to upgrade to macOS 11.0+ and see if the issues are resolved. In virtually every case we have seen resolution.
The same goes for the "inverse" of running Illustrator 25.0 on macOS 11.0+ with a plugin binary built against the AI25 library - getting these folks to either stay back on macOS 10.15 or to update to Illustrator 25.3 and get a plugin binary built against the AI25.3 library.