Photoshop saves alternative / incorrect file number
I have started having a problem when saving files in an alternative format, specifically: saving a TIFF as a JPEg or saving a Tiff as a JPEG via 'export as a JPEG'. Photoshop sometimes (but not always) substitutes the file number of the first file in the JPEG folder to which I am saving. Because this doesn't happen all the time I have to keep a sharp eye of the file name in the 'save' box in case a substitution has been made....sometimes I forget and miss the substitution....it's really annoying. This is a fairly new phenomenon...can anybody advise why this is happening (or what I have started doing wrong)...and how do I stop it.
Thanks
Windows 10 Home.