…specifically the Yii2: ORM and DB module and transactions. The Yii2 $I->seeRecord() & related methods do NOT use the same connection ID as the DB module. So doing actions such as importing fixtures and executing actions via the ActiveRecord abstraction happen on the frameworks connection.
Trying, then, to do actions like $I->seeInDatabase() and related DB modules actions will fail, almost always. Why? The Db module uses a separate connection, as defined in the suite.yml (acceptance/functional/unit) files.
So, from here out I will not be using the two modules together. Either use Yii2+Fixtures or use the DB+dump.sql. Both, together, is problematic at best.