Opus 12.3 included the change Jon talked about above:
When using the metadata panel to set an image's last modification timestamp, Opus now sets the EXIF DateTime tag to the same value automatically.
If you're having problems with tag inconsistencies between programs, please upload some example images, with versions of the files before & after changing the tags, and details on what to do to the files and what to look for for what's wrong in different programs.
Also make sure the other programs are looking at the EXIF tags and not something else which may also exist within the file and need to be cleared. There are a lot of tag formats, as well as a lot of overlapping tags, as image tagging in general is a jumble of different standards. For images, Opus edits EXIF tags (in general), which are the most common type.