When using Save As (PS 23.0.0 and Monterey 12.0.1 the focus is not in the file name field. When I click into the field and type one character the filename extension (i.e., .jpg) gets added and the cursor goes to the end of the extension so I end up with filenames like "p.jpgrestofname"
Seeing the same thing in Lightroom when printing to a JPG file (screen shots attached).
John