I ran into this today and thought I would share.
After installation of Service pack 2 for ArcGIS Desktop 10.8.1 you will end up with version 10.8.2(10.8.28388) in the splash screens as expected.
However, if one looks at the internal version number of the ArcMap.exe, ArcCatalog.exe, etc. they are actually shown as version number 10.9.1.28388. This is contrary to everything ESRI said in 2019 and 2020 😊. So, ArcMap 10.8.2 is actually 10.9.1 under the hood.
This explains why ArcMap 10.8.2. can publish services to ArcGIS Enterprise 10.9.x -even though Pro is now preferred.