It’s in “Edit Game” -> “Distribute” -> “Build Settings”. The one I was referring to is:
[ ] Enforce unique versions per channel
Pushing a build will fail if the version already exists in a channel
But I also saw the other tickbox:
[ ] Enforce semantic versioning
Versions that aren’t of the form a.b.c-suffix (where -suffix is optional) will be rejected.
These were the only reason I could think of why a build would be rejected. If both are un-ticked, then maybe something else is happening.