I suggest using Report an Issue to document that bug. It sounds very strange and one with potential problems for others.
If your users use passwords to access the file, you can set up your "lock" to lock invoice records.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
I thought I researched that in the beginning of locking down my solution.
Question: Does the limiting access on a record-by-record basis prevent seeing the record?
The users still need to be able to access the previous records but not change the data in selected fields, conditional to the boolean that places the zero in the lock field.
You can use a "lock expression" to limit which records may be viewed, you can also use such an expression to limit what records may be edited.