Sounds like you figured it out.
If it's for Service Catalog you can use the User Criteria Debugger tool to see where it is failing:
https://docs.servicenow.com/bundle/orlando-it-service-management/page/product/service-catalog-manage...
The user criteria cache does have key based entries for specific users, but, AFAIK the API is not public to directly access the cache. The user criteria cache entries relating to that user should be getting updated automatically if there was a change to group membership. I think scripted user criteria aren't cached in the first place, so if it is scripted that shouldn't be the problem. Unless for some reason you have set glide.user_criteria.cache.scripted to a non-out-of-box value. That should never be done unless every single one of your scripted user criteria are totally immutable for every user.
