I just made the switch from the Muon Version and after making sure the settings are as I like them I noticed that YouTube Videos are still playing automatically (even though I have disabled Autoplay on all websites).
I am using Brave Beta Version 0.56.5 from the Brave repository on Linux Mint 19 Cinnamon (64 bit).
The directions you gave here don’t work. Here’s the version I’m using.
Version 0.65.121 Chromium: 75.0.3770.100 (Official Build) (64-bit)
I go to autoplay like you said. The options I have are Ask (default_, Allow and Block. Allow is selected. I choose block and it stays on allow. I can click on Ask(default) and it changes to Allow(default). I can’t change it from Allow.