Fields like Category or Subcategory fields have many choices. when these fields are used in list condition builder by giving a specific condition like 'Category is Hardware' and run the filter you may observe filter breadcrumb changed to 'Category=Hardware' and on expanding the filter condition shows as 'Category is None'.
This is due to 'Label' and 'Value' of choice is ending with a space character at the end of the text.
This is causing the query no to recognize the filtered term and showing 'None' in condition builder and corresponding choice field column in the list may also show text in 'blue' color.
To fix this issue, remove the extra space character at the end of the 'Label' and 'Value' for the affected choices.
Affected choice field have space character at the end
Label: 'Hardware '
Value: 'Hardware '
Clear the space character and save the field which will resolve the issue and query will not return None value.