Do you mean that you are able to watch the content, but if you close the tab or browser, then reopen it and visit Netflix, you’re logged out? Or is the behavior different?
Does the Spotify site still load, search and function normally but only actually clicking “play” does not work? Or is the behavior different?
Lastly, I’d be curious to know if you open a guest profile in Brave (Menu --> More tools --> Guest profile) and login to either site, do you get the same results?
I found that I still couldn’t solve the problem, so I’ve given up using Spotify here. However, I don’t want to keep subscribing to Spotify. So I found AudFree Spotify Song Downloader to help me download songs to my USB drive.