I believe the root of the problem is that there is no de jure standard way to rate photos using Exif. The only Exif tags I know of which relate to this are:
0x4746 - Exif.Image.Rating - Rating tag used by Windows
0x4749 - RatingPercent - Rating tag used by Windows, value in percent
but despite the tags existing the values they contain follow no standard I'm aware of.
Let's try something. Here are three photos, I set the Exif rating tags manually using ExifTool: one uses the Rating tag, the other uses the RatingPercent tag, the third is without these tags straight out of RT. Does your "other software" show the ratings at all? https://filebin.net/0ly7t8s8oci31kqj
The 5-star rating system seems to be a de facto standard, though how its implemented under the surface probably varies. It seems that:
- Exif is generally not used for ratings,
- IPTC might be, but it has been superseded by XMP so no point wasting time on obsolete technology,
- XMP is used for rating.
The XMP spec has this to say about
Issue opened: https://github.com/Beep6581/RawTherapee/issues/3728