This is a bug that has been occurring in PSU for a long time and I can confirm it 100%.
There are several (RAW) editing software products out there that do a bad job in managing metadata. Unfortunate but understandable as they are editor and metadata is of less relevance for them. But PSU (and other DAMs for that matter) is all about metadata and if something is wrong in the metadata then the DAM is the place where you will typically be confronted with this.
When it comes to orientation, some editing tools (I'm not going to name them here) fail to write the correct orientation tag in the metadata at all and some write the correct orientation in Exif but fail to write the orientation tag in XMP if there is XMP, leaving you with misaligned metadata.
In general it should be safe to assume that if you see a wrong orientation in PSU that it there is something wrong in the metadata. PSU loads the file and applies the orientation that it finds in the XMP. If there is no XMP then it reads it from Exif (PSU *always* uses the highest level metadata first, which is XMP). If there is existing XMP but the XMP holds no or wrong orientation tag compared to Exif then you'll see such discrepancy instantly in PSU.
If you experience this then:
1. First use "Metadata -> Convert metadata to XMP" to make sure that your XMP metadata is the same as the Exif (many tools fail to update XMP)
2. Manually update the thumb
3. If the orientation is still wrong then use the rotation button to correct it.
Since these steps first make sure that there's no discrepancy in the metadata you can rely on the orientation being correct at all levels after these steps.