Just to be sure, installing the CA from that MITM box didn't work? Because that should be the generally recommended solution and I can't see why snap would have a hardcoded CA list separate from the system. If that didn't work, it's indeed a bug, but a rather weird one; definitely worth posting to the bug tracker.
The CAs are hard embedded in the core snap. They're pulled from some specific package when built, but snaps themselves are immutable. We attempted to overwrite it in several different ways, but the OS is just simply mounting these folders from the core snap (which is immutable), and then marking those mounted paths them as immutable.
Just to be sure, installing the CA from that MITM box didn't work? Because that should be the generally recommended solution and I can't see why snap would have a hardcoded CA list separate from the system. If that didn't work, it's indeed a bug, but a rather weird one; definitely worth posting to the bug tracker.