Hmm, two strange letters for shortcuts in the Menu “Glyph > Update Metrics” and “Update Metrics for masters”.
Glyphs 3.0.3 (3086) and Big Sur 11.4.
macOS is using German language and “Disable Localization” is set in Glyphs3.
When I turn on localization (German Text) shortcuts working and no strange letters to see.
In Glyphs under preferences, no shortcuts are set.