If an address group is missing, the places referencing the object must be missing too, likely policies, at the sandby unit. If that's true, I can think of two options, or three.
1. do the same resyncing from scratch
2. take the entire copy of primary config and modify those unique parts like host name, management interfaces, HA override, etc. Then upload to the standby.
3. remove the missing part including the dependencies from the primary, then re-configure them again at the primary. Out-of-sync doesn't mean any config changes won't be copied over. Likely that part is still working.
But you need to think about how it happened. Like the script copied and pasted when you address the group might had errors and you had to manually fixed it, or something else.