I experience the same thing. I log in: Character starts spamming Left Hand (LMB), I change characters, Character starts spamming. Doesn’t happen when I return to the Sanctum for the game world.
It’s been around since forever and therefor I was unsure if its been reported.
Before arriving at the Sanctum after starting the game and when switching characters, is always a UI click right before.
My Guess: This mouse click is not fully consumed by the ui and somehow locks the mouse to a mousebutton down state for the game. Clicking again ends the LMB spamming.
I don’t think thats a bug, I guess there is a script breaking the warp blocks for you to open the warp.
You are just not supposed to see the damage numbers in the UI.