Peace likely behaves buggy because its a minion. I don't think effects like fade and venom are vanishing or you'd hear a lot of reports/complaints about it.
I'm gonna play around with Peace for a bit and add in whatever info I come up with about what causes the Valk to disappear. But as has been demonstrated and mentioned there are likely many ways that the Valks poof that seem to occur when the Valk or your character is effected by a skill/curse.
If anyone else wants to add in their findings that'd be great. I'll just get a table going:
Caster Cast Upon Skill
Monster Valk Amp Damage (end duration)
Any Class Self Town Portal (after going to town)
Paladin Self Holy Shield (end duration)