I am not sure if this was fully intentionally. The code just happens to cause this.
The tag writing only handles “performer:role” tags, and those get saved as TMCL frame (or IPLS for ID3 v2.3).
There is no handling of “performer” (without colon) tags, so those get the standard treatment and end up in a TXXX frame.
If my memory is correct in one if the 2.x we had some fixes for performer tags, unifying their behavior when specified without role. It could be that this changed tags without role from being called “performer” instead of “performer:”, and that could have changed behavior here. If that’s true I would consider the current behavior a bug, or at least unintentional.
In any case I think we should change the behavior and make sure they all get saved to TMCL frames. I think even empty role would be possible, if not we could use something generic as suggested above.