I’ve found the rainbow portrait few years ago and now I’ve discovered … it is missing into my cosmetics rewards.
Any idea?
have I to restart Sir Williams farming?
Only one thing, the character that I’ve used to farm the portrait has been cancel (no any particoular reason), no any other info.
Are you on the same region you found it on? American v. Europe v. Asia?
I’m european region and always played under Europe realm
On your hardcore or softcore heroes? I don’t play hardcore (potato computer would kill me) so not sure if cosmetic rewards are shared across both game modes? 
softcore player … mee too potato computer). This think I’s making me crazy … but time to time I’m wondering for Mr. Williams … lovely
Cosmetic Pets and Portrait Frames are global. (Available on both Normal and Hardcore). No matter which mode you earn them.
I’m not sure if the image below will help you locate your Rainbow Portrait Frame. And I may have some different Frames than you. But, if you already earned them, they should be listed shortly after the Base Portrait Frame.

Young fellas thanks a lot for your efforts. I’ve already checked (I don’t know how many times) … no way, no portrait. Maybe I’m really crazy 
I know you aren’t talking to me, but you’re welcome nonetheless. Nothing I can think of that would make it disappear from your collection, though.
Make a ticket. Perhaps the support can help.
Maybe they changed something behind the scenes that made a bug appear that deleted cosmetics from some accounts. Anything seems possible.
I never had any issues, LOL I don’t know how to start a ticket … poor me
I suppose you could do a search and find out, instead of throwing up your hands in self-pity and just giving up.
That is what I would do anyway, but to each, his own.