Problem with Suggestion charmed monsters and other spells

I have noticed that whenever I cast Suggestion on a monster and it joins my party as a character something strange happens if I or anyone else casts a spell on it, or an area spell that affects it: the Suggestion spell is immediately cancelled! What is going on here ? As it stands If I cast Suggestion on a monster and it gets injured I cannot cast a Cure Spell on it, for example, though non spell healing seems to work. Something needs to be done about this I think.