Schematic symbol library file is locked by a design and not released after closing capture

When editing schematic designs in capture, our shared schematic symbol libraries do not normally get locked.

But for one design, one of the ligrary files get locked and it does not release the lock when exiting Orcad capture.

Whichever user opens the design, a lock file is created, in their name, in our folder of shared schematic symbol libraries.

Closing capture does not delete this lock file. The user then needs to navigate to the folder and manually has to delete the file.

It is only for this one design, it does not happen for any other designs.

I cannot find any reference to this library file in the project, except in the cache and other libraries are in ths cache and they are not affected.