OK, I think I *might* have finally got to the bottom of this. It seems on rare occasions, if you pixel-perfectly jump sideways on to a platform, sometimes the platform behavior would accidentally detect that as an upward slope that you're walking up. By tweaking the handling of slope detection, I think I have fixed it. You can try the new platform runtime.js here and let me know if this resolves it for you:
I was hoping to do a stable release next, but I would really prefer this to have thorough testing in beta before putting the fix in a stable update. We've had small tweaks like this break the platform movement for some people before, because it's really hard to get right for all rotating gravity/jumpthrus/etc. that people use. So even if this works I might *not* release it in the next (stable) update, but put it in the first beta after that. (In the mean time you can use the runtime.js above.)
Let me know if this works for you, and especially if you notice anything working at all differently with slope handling.