Session message disappears if db transaction is enabled - bug?

"flash" is irrelevant here - this will happen with any type of session data when you use the database driver.

The problem is, when you share the database connection (and that may happen even if you try to avoid it, it's just how PHP works), a transaction started for your own queries will obviously include the session library's queries - you can't avoid that.

And the library can't avoid it either because there are two strategies to take - discard current state of the transaction or commit it unconditionally. Both ways would cause a problem for somebody.