In my previous Crusader Silver attempt, something broke with momentum. I cleared a bunch of popcorn but the bonus remained stuck on 44% (IIRC). I'm not entirely sure what caused it, but I had just gotten my first overheal from Scholar's Pact, and right before I'd used one fireball on the boss which naturally afflicted me with weakening and corrosion from the retaliation. No idea whether that's relevant, it's all I can recount.
I realise this isn't much info... I'd try and figure it out further but I really have to go sleep (DD steals enough sleep from me as it is
) I'll try and reproduce this again tomorrow, I've been bashing my head against the challenge for a while now (seriously, the Explorer's Guild challenges feel much harder than the other class challenges, or is that just me?)