My mod has a branch for 2.0 and 2.1. I recently pushed an update to both branches, releasing the 2.0 support patch after the 2.1 patch. The changelog viewer in the mod browser is showing the 2.0 mod versions history even though I'm on the 2.1.20 and the latest version shown for the mod (1.9.9) is for 2.1. These 1.8.x versions of my mod are for Factorio 2.0.
To be clear, I expected the changelog to show 1.9.x versions of my mod because those are for 2.1, as I'm on 2.1.20.
[2.1.20] In-game changelog viewer doesn't show the correct versions for the branch
Re: [2.1.20] In-game changelog viewer doesn't show the correct versions for the branch
This is the same behaviour as the mod portal (and the game presumably uses its api for this). And its why you should upload older version first, then newer version. As a workaround you can take down and re-upload the latest 2.1 version.
Whether it should be this way is not for me to decide, just a suggestion.
Whether it should be this way is not for me to decide, just a suggestion.

