Testing multiple major versions how?

We have two majorapp versions

1. x The version in the store

2. x The next major release

Both 5.x and 6.x builds are visible in App Store connect (TestFlight)
The status for 5.x builds is always 'ready to submit' without any obvious way to submit the build to Appstore connect testers.
It is possible to add external testers, but we need our Appstore connect testers!

Is this a TestFlight bug/limitation?