SharePoint List Person or Group Column Stopped Functioning Correctly

Hi, 

I have a custom list on a Team that we have been using for about 4 years to review and track invoices. One of the columns is a people/group column called NotificationRecipient. Once an invoices has been marked as reviewed, a workflow is triggered that sends an email to the Notification Recipient.  Some of those individuals are employees at my company, but several of the individuals are clients who are authorized guests of the Team. This whole thing has been working fine for years.  Just a few days ago, suddenly, the individuals who are clients are no longer coming up when we try to populate that column.  I haven't made any changes to anything. I've checked Teams, and the individuals are still part of the Team. Does anyone have any suggestions on why this might happen, and what I can do to fix it?  Thanks in advance.