I rather think ist because of different program Version we're using... your GUI is looking different to mine...
following screenshot Shows the step I've to do to change the exposure time value with successful saving 1/60 was original value I changed to 1/600...
I use the Version v18.104.22.168 currently newest Version is v15.5. from this site:http://u88.n24.queensu.ca/~bogdan/
so I did update/replace it by downloading the exiftool-9.22.zip and the exiftoolgui515.zip:
I extracted the exiftoolgui515.zip, I got:
in this Directory I copied the exiftool.exe
than I made a link on the Desktop to C:\Program Files (x86)\exiftool\exiftoolgui515\exiftoolgui\ExifToolGUI.exe and I run it as Admin (without it cant save the config file on closing exiftoolgui)