Just wanted to share this since I couldn't find this solution when I searched but the Garmin support team was able to help.
Some of my dives wouldn't show up in my iOS Garmin Dive app but I could see them in Garmin Connect.
The solution was pretty simple: connect using Garmin Connect web, edit the title of the dive, and save. (Maybe saving is automatic? not sure)
This solved this issue and dives showed up in Garmin Dive app.
Hope this helps!