The bandit is... still hostile, unfortunately. Diplomacy was still just a
bit buggy. By which I mean impressively functional for something so incredibly non-functioning. That which says hostile,
is.
E: Also, yes, if the bandit was actually your ally and you didn't have complicity disabled (Note: Disable freaking complicity. Allies and alignment do not play well together, at all.), it would quite promptly crap all over your alignment (yes, annoying any gods that troubles) until something killed it.
Also RE: Frumple, I'm not sure what you mean. I've never had that happen Mages are tough, yes, but I've never seen one with more than a handful of persistent enchantments. Unless I wiped them too fast to even notice such things, heh. Note I usually play a climbing lizardman monk with incredible speed/attack speed so anything without armor or damage negation [or specifically countering spells] gets shredded easily.
Yeah, kick on wizard mode and challenge or nightmare difficulty, pop down to 10+ depth, and hit a caster with a high powered dispel wand. I've seen literally over a hundred sustain effects drop at once, before, iirc (It may have "only" been a several dozen, I'unno.). And this was in latest version (H4), yes. Casters will generate with literally every sustain they can cast, cast. Lower level stuff, that's not that big of a deal, but with high CL templated things (or, y'know, cast-capable dragons) it gets silly.
Though nah, you were probably just getting lucky. When you melee a high CL caster that's got its sustain boner popped, you will know it.* Because somewhere between a fifth and half your life will disappear every time you try to give 'em a love tap, due to the stacked retaliation shields. Also they will probably have more HP than you. And you'll be fighting them in a fog bank.
* Actually, you'll know it long before then because you'll see the aura effects from a room away, once you start getting deep enough.