This is 100% sure an issue with the game. As stated in the notes, if you experience any difference in later versions, try the same fixes. Minecraft has never changed the actual sensitivity, but have had issues with FPS, vsync, polling rates and conflicts with keyboard input and sensitivity, which this is likely related to also. If you can try a tool like this in both versions you should see that they are exactly the same (granted vsync is off, and expect a few tests to be slightly off due to packet registration issues in the game), any difference you experience is basically down to a bug in the game that is inconsistent and impossible to account for.
Also, the difference between the old pre and post 1.13 calculations was just a slightly lower base sensitivity (about 1.3 % lower) that was so insignificant that it was barely measurable. It turned out to be down to packet losses in the old versions, which again doesn't actually change the sensitivity so having it as a separate calculation was wrong. The base is the same, and how the bug is experienced will vary a lot.