Well that confirms what I found, using MainStage's 'Assign and Map' function works for all Axiom controls with B-5.

So I guess if anyone has issues in MainStage using the midi learn or assignment features inside B-5 the simplest way to overcome these is to use the MainStage sequence suggested in my post above.

MainStage 3 | Axiom 61 2nd Gen | Pianoteq | B5 | XK3c | EV ZLX 12P
Hoping everyone stays safe and well and sees this through to the other side