I haven't had time to look into it in detail yet, but I just did some testing of my website with some JS, and it works in 15.5, but fails in 16.3.

Maybe someone knows something from already having come across it?

The main thing the JS does is set a short timeout, which prompts the user for a username, generates a key for them, stores it in LocalStorage, sends an xhr request, and goes to a new page.

Somewhere in the middle of this it just stalls, and I haven't had a chance (or equipment) to do detailed debugging on it.