After testing, the problem appears fixed in 1.0.9 Build 240521 I enabled OFDMA+MU-MIMO and it stayed enabled, did a couple reboots and power offs and it is still showing enabled.

New firmware out for US, I haven't tested it yet, no specific mention of a fix for this issue though. Perhaps it has fixed the issue? Archer BE550(US)_V1_1.0.9 Build 240521 Published Date: 2024-05-31...

Same issue for me but with a bonus of dropping wifi on Samsung devices no matter what combination of settings, channels, smart connect, etc I had the same issue on the previous firmware. Fold 3, Fold...