Here's what happened:
From top of my head I could not recollect from my experience why the managed solution showing your custom fields or is it intended.
Though it should not remove any of your custom components when you’re deleting/uninstalling the third party managed solution, I cannot comment on any third party about perfection.
I would recommend you to do a dry run by installing the managed solution in any sandbox environment and testing yourself by deleting it. This way you can be sure.
Typical managed solution should not remove any of our customization when uninstalled. They should only wipe out their components.