When trying to set up a custom background for my character, I found that the proficiencies often don't show up on the finished character sheet. Sometimes, I do get it to work, but in my recent attempts, it works infrequently at best (I got it once with a different "Proficiency/Language Choices" option).

EDIT: I tried again, and after trying to return to my character sheet, I got an error. When I closed the page and reopen, my custom background had properly filled in. Not touching it again now that it's working. However, the system still seems buggy, as it was definitely exhibiting inconsistent behavior.

It can take up to 20 minutes for some changes to occur due to caching issues. Custom Backgrounds are among the most consistently problematic for me too. Now if I want to edit a custom background - I change it and then close the tab. When I check on it a while later it's usually showing correctly.

Yea, this is annoying. 
I really just want to use this each time instead of searching through all of them.