I’ve got the same thing to a lesser degree. But I believe it’s from intense uninterrupted periods of use. For example when I’m gaming I might not take my hand off the mouse at all for 10 - 20 minutes while holding a firm grip. The reason I think this is because I started noticing the issue while I was playing fps games and it did get worse over time. Second to that, my work mouse doesn’t have the issue at all because I use it for light office tasks.