As @tobiaseigen pointed out, Google tells us exactly what causes a message to be “un-muted” in the conversation:
Muted conversations will only pop back into your inbox ready for your attention if a new message in the conversation is addressed to you and no one else, or if you’re added to the “To” or “Cc” line in a new message.
(emphasis added)
… both of which are currently true for Discourse topic notifications.
See also this feature request, which could “fix” this issue: