When leaving the PeoplePicker after entering single space, it automatically resolves to "AllUsers(Windows)" or "NTAUTHORITY\AllUsers".
Why this issue happening? Please give your solutions/suggestions to solve this issue.
Does this issue happen on all people column or just a specified people column? Test in other list or other site.
Try to delete the people column and re-add it then check the results.
Make sure if there are some search rules that make this issue.
Go to CA > Search service application, do an index reset and do a full crawl. Then check the results.