AetherWart

This would definitely make sense, but I want confirmation on this, since if it's a bug and it gets fixed it might open up some opportunities for redstone machines that rely on consistency in the direction of projectiles thrown by a player.

I know bows, dispensers and droppers have this randomization, but I've never heard anything about player-thrown projectiles.

Can confirm, the issue is fixed for me as well. Thank you dev team! Anyway, can all of you please test if you still have it? Maybe reupload the skin to minecraft net and the launcher to be sure.

Just had a thought that might help - If the Minecraft Wiki isn't lying to me and the Alex model was added in 1.8 pre-1, then switching between a version before the addition of Alex and one that's after it has nothing to do with the issue. I, and probably most Hypixel players, only switch between a newer version and 1.8/1.8.9, which are after the addition of Alex as well - and we're still getting the issue.

So it seems like the "switching through versions" that was reported by the original reporter has nothing to do with the issue.

I wanted to see if this is really that widespread - so I logged onto Hypixel, a server that uses 1.8 PvP so it's likely for its players to go between 1.8 (no Alex model) and a later version (with Alex model) where they play in singleplayer.

My expectations were confirmed - lots of players with the same skin problem. This is a big thing.

Sorry for the follow-up, but it seems like some of the images I attached don't show fully (at least on some resolutions - so open them in a new tab if you're using them to try resolve the issue.

I have the same problem. Here's my experience with it.

I last updated my skin a few weeks ago. The problem only appeared today.
I go into a singleplayer world, no texture packs, no mods (no OptiFine), 1.15.2, and what happens is in the 2 photos I attached (using F5 to look at myself from the back). I also attached the skin file.
I went back and reuploaded the skin to both minecraft . net and to the launcher, waited half an hour, no change.

Anyway, hopefully this gets resolved soon and I love seeing how the community jumps on stuff like that to deal with it as soon as possible. Good luck, and hopefully my images help.
I also attached an image showing that when I go into the "skin" section of my Minecraft profile, I see the model selected being "Classic", even if I already uploaded a skin there after selecting "Slim". This might be unrelated, but worth a look IMO.

This also appears to be happening to me in all versions of Minecraft that had the Alex model, even 1.9. This also might just be me, but it is happening to me.