When I key bind my mouse buttons to say the fireball, save and restart the game. I lose that binding.
Minecraft 1.12.1
Forge: 14.22.1.2485
Mod: avatarmod-a4.6

hmm
Try updating and see if it still happens

I tested 5.3 and it is holding the bindings now. Sorry about getting back so late, I've been "out of the scene".

Well I just got back on after two days and the bindings are not active again. Active as in retaining the key I set.

I am running single player and am using Forge with Liteloader, but no keymapping mods.
1.12.2-LiteLoader1.12.2-1.12.2-forge1.12.2-14.23.4.2705

Hmm that's weird!
-What forge version are you using?
-Are you on singleplayer? Multiplayer? A LAN network???

Ok tested it out myself- Definitely a bug. Gonna bet mahtaran to look at it while I'm gone.

Try downgrading to forge version 1.12.2-14.23.4.2611 or 2555!
Also, if that doesn't work, try removing LiteLoader.

Known bug.

Did that ever get patched? Because I'm facing the same issue.