The release of 20w21a said it had fixed the bug where wither skeletons don't spawn in wither roses but i'm not getting wither skeletons spawning in logicalgeekboys wither skeleton farm (link to world download: https://www.youtube.com/redirect?redir_token=hyZdTPIjHn9Dad1F5VJAVJAMCix8MTU5MDA3OTIxNEAxNTg5OTkyODE0&q=http%3A%2F%2Fbit.ly%2Fwither-skull-farm&event=video_description&v=OxUIJcTcokw) it worked fine in early versions so it should be working now. The issue could be that it's no longer a fortress, i didn't see blazes spawning when I removed the wither roses only zombified piglins but i didn't test that for very long. 

Edit: After longer testing it's probably that the area is no longer considered a fortress which i still think is a bug.