The BB threads are designed to unthread as you pedal forward in the event a bearing should seize up so that you won't break an ankle or other body part.
If the BB is designed for an eccentric insert to change from Ashtabula cups to three piece cranks, then there should be a way to lock the adaptor into place as in a pair of clamp bolts for each side of the eccentric insert.
If that is not present you will need to change the insert to one that can be fixed in place. The fixed ones centers the insert and the chain tension is adjusted by chain length. HTH, Smiles, MH