That is why I am posting it here as it seems to be something with your latest update which is impacting many php based themes.
To be clear, there are 8,588 themes in the free directory alone, and an uncountable number of commercial themes like BuddyBoss.
There’s absolutely no way that the WordPress developers can test against all of them, which is why we run a length public beta period asking theme developers to test their one or two themes with WordPress 5.8 before it’s publicly released.
This particular beta period ran for almost 2 months: https://www.ads-software.com/news/2021/06/wordpress-5-8-beta-1/
I’m sorry BuddyBoss decided not to test their theme during that time, but please at least pass along the solution you found to them.