In mysqli phpBB driver, function _sql_transaction()  does not perform BEGIN statement in transaction begin case but only disables autocommit.

That may lead to unpredictable results in some cases as there's no actual transactions got commited.

Recently faced a case with bots deletion via ACP where all the bots were deleted from users table but not from bots table which caused a bunch of hard-traceable PHP and HTTP errors. There's no proof it was caused by transaction issue but all those bots deletion queries are put into a transaction within the code.