The same here.
I found that it did work with the standard theme Twenty Twenty.
So it must be a problem with the theme we are using.
Contact the theme maker.
Thank you for your feedback. Highly appreciated
Please see the screenshot of the setting and activity stream from the dashboard.
Post Comments are enabled. https://nimb.ws/CoSGJU
But in the admin, the dashboard activity stream says Replies disabled. https://nimb.ws/sWyvdR
I don’t have that “replies disabled”.
So here the replies give the impression to work but they don’t.
I think you should ask your theme developers.
Mine are working on it, I hope.
But of course first try to disable all plugins except BuddyPress and test, so you know that no plugin is the culprit.