Seriously, can someone explain the issue to me like I am five? I'm not saying anyone is wrong or making it up but I've not experienced it so would like more information.

I got almost endlessly heavied/stunned last night by a coordinated group of (I think several dragoons - I saw Spine Shatter) & didn't develop resistance. Okay, it was about 5 times but it felt endless in a fight! When I got on my mount I didn't have slow/heavy disappear as others have stated, it was still there. The same applies when getting off a mount, the status remains.

I wonder if part of the change is that pets used to remain with you when mounted so could affect any statuses if they had the ability (eg Selene). Maybe the change to pets & mounts had an unexpected effect resulting in what others are seeing.