Hello @laura_p 

The most likely cause is browser/instance caching or an Active Dictionary Override at the Knowledge Base table level (kb_knowledge).
Clear your cache (Browser cache and ServiceNow Instance Cache using cache.do in navigator).

Verify if there's an active Dictionary Override on the kb_knowledge table for the Valid to field (valid_to) that might be overriding the OOB settings, especially if the customization was done via an override initially. If present, delete or deactivate the override.

Note: I tried to change in my PDI with dynamic default value and latter switch back to OOTB version and it work no other config change was needed unless it was done on your instance.

If my response has helped you, hit the helpful button, and if your concern is solved, do mark my response as correct.

Thanks & Regards
Viraj Hudlikar.