How naval treaties handled an accidental battleship loss

If a treaty signatory lost a battleship by accident, did it have to build a replica?

No; accidental-loss clauses allowed a new replacement design.

Naval treaties included provisions for replacing a capital ship lost by accident. The replacement did not have to reproduce the destroyed vessel. A signatory could build a new treaty-compliant design and retain its permitted force entitlement. An accidental loss could therefore update the fleet without reopening the treaty allocation.