Do you know if they use the same OD and pitch as Heli coil? This is very strange the heil coil is so loose. Almost make me wonder if someone used the wrong tap ( 8mm x 1.5? )
I have never encountered this in over 55 years of turning wrenches. Even if the time-sert goes in I would first THORUGLY clean the case threads, and install using an additional locking compound or possibly even JB weld.
If one coats the installation tool with vaseline it will prevent the JB from plugging the female threads when removing the tool. Leave if sit the full 24 hours for curing. If the bolt will not go in easily, slowly run a SHARP tap in doing the 1/2 turn in, 1 turn out. If it's a blind hole you may need to follow up with a bottoming tap.
Another possible option would be to press fit a plug in depending on the location and how much material you have to play with.