I too have been using Word for many years and consider myself to be fairly experienced.
I came across this extremely frustrating and time consuming anomaly today.
I find it disgusting that Microsoft can make millions from us consumers and then treat us as idiots while denying the issue exists. It is clearly new and was introduced with the June 2019 update.
I am an amateur macro writer and wrote the following simple macro:
Sub FindItalics()
Selection.Find.ClearFormatting
Selection.Find.Font.Italic = True
Selection.Find.Execute
End Sub
If I run this macro, all instances of italics are found, even when I click 'Find Next' or press Shift+F4.
However, after I use F&R for anything else I have to re-run the macro to find all italics again, otherwise it reverts to the issues you describe.
I hope this helps until Microsoft admit there is an issue, investigate it and fix it.