Hi,
I usually draw up my characters in Procreate, export as .png and then c&p to trace in Glyphs. That system used to work just fine in Glyphs 2, and still does, but not in Glyphs 3. Something does paste as the base character disappears. I’m sure it worked initially as that’s my usual process, but not having done much in the last couple of months I haven’t noticed when exactly the issue started.
Consider using the universal clipboard instead of Preview.app. Make sure both your iPad and your Mac are in the same WiFi, then you can just copy and paste directly from Procreate on the iPad to Glyphs on the Mac.