I was just walkin around and it vanished into thin air. It doesn't appear to be anywhere, I've looked around with spectator. I also couldn't figure out how to take it off. I'm using first person model, which may affect this? And also Carry On.

Bro it's in my boat over 60 blocks away wtf?

Still can't take it off.

No, it's been stable since. And I figured out double shift, I forgot some mods use that instead of shift + right click.

I'm not sure what could have caused this. It sound like there was a desync between the server and client, where visually it looked like you picked it up but in reality you didn't and it's still in the boat. Does this happen consistently?

Shift+Right-Click actually makes a lot more sense than double-shift. I might just make it that instead. Anyway, glad it never happened again