I may be going insane, but when you first introduced relations type fields (pivots) there were two options

The multi-select fancy combo box (which is what we get now) which is awesome

but there was another option where all the options were laid out like.. for want or a better term.. radio buttons where you just clicked on what you wanted and they changed colour

We now only seem to have the combo box options.. or maybe I am wrong and this was only for enumerations.

Is this an error, or am I making stuff up

I think it was only for enums, but not sure actually....

I think you are correct, i likely just had a tiny list in an earlier version before I moved to n:n

It was only for enumetations in facts, not for n:m
The reason is simple, radio buttons do not allow multiple selections.