Filtered lists and HIllbilly tabs

A predecessor of mine created a master list for staff and used hillbilly lists and filtered lists to sort the list. However, a bit of a bizarre has happened and for some reason the list is now showing people leaving the whole month of July 2019. The administrator added that consistently. The views are good and show every other month. Only not in July

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Has anyone ever seen this problem? I am using SharePoint 2016.

thank you in advance