Bartender 3

We are aware of one cause of this in macOS, this can be fixed by: Ensure you only have a single copy of Bartender 3 on your Mac, you will want it to reside in /Applications/.

Use the following command in a Terminal window. The command is:

xattr -d -r -s com.apple.quarantine '/Applications/Bartender 3.app'

It seems for reasons unknown that the quarantine flag, set by when you download something, isn't being removed sometimes. We don't know if it's the case that it isn't being removed properly or if for some reason the user isn't seeing the normal prompt.
Anyway, if that flag is present macOS won't allow you to run Bartender 3 at login as it's trying to protect your system. The command tells macOS to remove the flag and then you should find Bartender 3 runs at login.
The reason you want only 1 copy of Bartender 3 is that macOS seems to randomly choose's which copy to launch, so this could be any copy on your system. If that version is still quarantined (never launched etc) then Bartender 3 simply wont start.