When a % is included in the display mask of a field, errors can result. If a user clicks inside a field containing that display mask, does nothing to the contents of the field, and then clicks the Save button, the errors can be seen.
The platform validation of percent fields fails when a custom mask including the % is used. As a temporary fix, the only fix available would be to remove the % from the display mask, which would require having a % as a separate label field next to the data entry field to be handled as a percentage. Moving forward, the platform validation of percent fields works correctly when a custom mask including the % is used.