dynamic paths using sub no longer working

Hello, great plugin. I am using dynamic paths with sub=”fa-username” attribute.
I was running Version 3.9.6.1 and it worked fine.
Upgraded to Version 3.9.7.5 and dymanic paths stopped working. In debug mode the path just shows the base path. Reverting to Version 3.9.6.1 works fine again and debug shows correct path (e.g. base path plus sub, in this case fa-username which is currently logged in user).

Any idea what might have changed that has stopped this working. I see there is a number of path santisation changes, not sure how that can be related.

Cheers
Tony.