Hey 357. It’s a common problem that has an easy solution. We have a natural tendency to pull the pick inward toward the body of the guitar on the upstroke. Whether one is taking the pick outward or inward on the downstroke doesn’t seem to lend the same issue as on the upstroke. So the main point is it can happen no matter if you are implementing upward pick slanting, downward pick slanting, or whatever else.
Pick depth is a big factor so make sure you are minding that, keep it shallow - use the bottom left edge of the pick tip on the downstroke, top right edge of the pick on the upstroke.
On the upstroke you want to make a conscious effort for an outward motion away from the body of the guitar, not toward. The angle I’m referring to is extremely small and there is a very fine line between a good angle and bad angle that gets you stuck. You can experiment by placing your pick on the bottom of the string and lightly pull upward. Play with the inward/outward angle at the point the pick easily releases from the string vs the angle you can continue to pull the string upward and you will see that fine line I’m referring to. Again, doesn’t matter if USX or DSX - can get stuck either way with a bad angle. Once you are conscious of this and fix the trajectory of your upward pick stroke, getting stuck will be a thing of the past.
Now my question would be is if anyone knows why this plagues so many on the upstroke vs. the downstroke. I’ll say it again - it’s an issue that doesn’t seem to care if you are using USX or DSX so one would think it would be just as prevalent on the downstroke. I think it has to do with the forearm twist angle naturally turned with the thumb angled toward your body/pinky away from your body - creating a down up picking motion that’s actually down/outward and up/inward. It’s that inward part that causes us to get stuck and fortunately easily fixed.