Tenacity Downed Rancor and Pigs resist Zader

Argarath

I have soloed with Zader team with TFP many times, but noticed only last few T7 rancor raid, rancor and pigs on occasion with tenacity downed resisted DOTS from Zader lead team. I know sometimes rancor can resist, but never have I seen no new DOTS applied at all when tenacity downed was active before recently. Has something change that we are not aware of?

Hskull55

There is always a 15% chance to resist a debuff

Argarath

I know but the chance to resist all dots and speed down. Like I said I have noticed this a few times now