Happens to me. Last steam version (PC), no mods. Very annoying.

Happens to me as well. When I died I lost all my upgrades on the modern axe. I'm on PS4 and this happened June 13, 2022.

Happened to me. Lost all visual upgrades to plane axe after booting up the game. December 28, 2022. PS5 version.

5/20/23 PS4 version: I upgraded my machete long ago and the upgrades disappeared in between sessions but this was only visual..to an extent. The stats still count but today I wanted to disassemble it to put on different upgrades and there's no option to. I can't put upgrades on it because it's already maxed out and I can't remove upgrades because the visuals aren't present. Also putting it on a weapon rack and grabbing a new one didn't fix it. My upgraded stats just transferred to the new one.

March 3rd 2024, happened to me aswell, that's only a visual bug. My modern axe was maxed out, then suddenly after relaunching the game, all the upgrades dissapeared. I can't upgrade the modern axe though (it was maxed out before) (see image for reference)