There is some confusion around when to create a new app build version.

In App Store Connect, once an app build has been sent for review no changes can be made to that build number. A minor build 3.2.1 vs 3.2.2 must be set for the next build created whether this is a TestFlight or release build. A way around this is to reject the current build, then it is possible to push another build to 3.2.1(1)

This caused me some headache before I understood it.