Navigation mesh failing to build

Occasionally the nav mesh bounds volume in my level just stops working and the only way to return it to the normal behaviour is to load up a second level and then reload the original level. I’ve tried messing around with the settings but haven’t found a proper solution.

Any ideas how I might work around this in a better way or stop if from happening entirely?

This is version 4.21 by the way.

the nav bound should only be in the persistent level (main - even if it is empty) .

The nav mesh bounds volume is already within the persistent level.