Mac Beta reverted itself to Production channel and won’t change back

The nightly would continue to be installed once you accept a nightly build. This would be true if 30 seconds later you switch to beta or production. Once on anything other than production you will remain on that build type until the next cycle change. Aka a newer Production is released.

I'm not saying you've not encountered a bug but that's how it's supposed to work.

We do not support moving between versions mid cycle. To do this you would need to take steps manually.