Are there any known exploits in Signal's encryption protocol that could lead to messages being decrypted, or any other compromising information being released?
No, there are no known public flaw in Signal at the time of writing of this answer.
Source: there was only one CVE for Signal (for Android), in 2018. The only one listed (fixed in version 4.25.0 the 14 Aug 2018) could leak photos attached to disappearing messages.
It's not so much a flaw but more an expected and documented but nonetheless surprising behavior: since Signal uses the phone number as an ID, it is possible to identify whether a Signal account tied to a specific phone number is a member of a group that you are yourself a member of.
This has apparently been used by Chinese government agencies to identify dissidents:
- Get yourself added to the "subversive" group chat.
- Check a list of suspected dissident's phone numbers against the members of the group.
[I can't find the article nor remember the details, so I'm leaving this as a community wiki. If someone knows the details, please feel free to edit.]