If a filter has one project selected and that project is deleted, the filter will be deleted to. (This is because the project is set on the SearchRequest when one and only one project is selected in the Issue Nav).

I am not sure this behaviour is correct.

I know the following is not correct:
If a filter has 2 or more projects selected, and both are deleted, the filter is not deleted and instead searches across all projects.

Should change both to not be deleted and return no issues.