RM521F-GL can't connect to 5G SA

I have a curious problem where my RM521F-GL can connect to 5G NSA, but not 5G SA.

I have confirmed that 5G SA is operational in my area, and the same 5G bands are selected for SA as for NSA, but when trying to switch to SA the module will just rotate through the bands one by one but never connect.

Has anyone experienced anything like this themselves or have any suggestions?

I have already tried re-flashing with the lates/only firmware for this module, with no effect.
This is using T-Mobile USA, for context.

Dear @nickkrewson
Did you try to diable NSA and confirm whether it register on SA or not.