List Created using Sandbox solution is not working now

We had a Sandbox solution developed using Visual Studio (empty SharePoint Project and then added a "List" definition/template). We had added some lists to this project and added all the lists to a feature and then this solution was activated on customer sites to create lists in SPOnline sites. There was no other code in this project, only some list definitions.

Since sandbox solutions are now deprecated, we deactivated the solution from customer site. However, after this step lists created using sandbox solution are no longer working and we get error when we try to go the list view, settings or any item form. Lists created directly from SharePoint UI continue to work fine in these sites, problem is only with lists created from Sandbox solution.

Since customers have items in these lists, we just want to use these lists going forward but without the solution that initially created them. Please let us know how this can be done. Again, note that we don't want to use any sandbox solution code, only the lists that were created earlier using them.



Additional information:
Generally, if we create a list item in any custom list created manually from SharePoint UI, item gets created in default content type i.e. "Item". In our list which is created by Sandbox solution, if I create an item it gets created in content type"ListFieldsContentType".

Now, I have deactivated my sandbox solution and then I am seeing an error "An unexpected error has occurred" when I open that list (this is true for the lists created by our sandbox solutions only).
If I activate the solution then again our lists are working fine (we have an extension for sandbox solutions for some tenancies).

Our sandbox solution feature created lists with "ListFieldsContentType" as a default content type and we have tried below things on a list but no luck.
1. Make "Item" as default content type
2. Add all list fields to "Item" Content Type
3. Update "Content Type" field value with "Item" in all list items in that list
4. Remove "ListFieldsContentType" from the List

With these steps we thought the "ListFieldsContentType" references are fully removed and we can use our lists without errors but the outcome is different. We are still seeing the same error message (Something went wrong, an unexpected error occurred).