I mean, the OP of this post is clear as anything - The GIF shows the issue, Sine behaviour + jumpthru on the platform, platform movement on the player.
It's certainly to do with the Sine platform skipping beyond the player as it moves up, right? - It can't be the player moving too fast DOWN because I think the platform behaviour has stepping and such to prevent this.
Just from the OP, no detail appears to be missing in this case.
Thing is, I wouldn't have reported this bug if I encountered it, because I have the strongest feeling I would spend time making a repro c3p file, fill otu everything, make a screenshot, upload it all, post it on Github... then I'd wait unknown amount of days, and eventually get a reply saying "Ah it's complicated" and that's it, no workaround offered or anything.
This happened with me posting a bug about the Drawing Canvas - an actual feature in C3 is to "Paste Effects into Drawing Canvas"...and it's broken for a bunch of effects. No fix, no timeframe like "This may be fixed in a year, or when we recreate the effects compositor", no workaround (ironically, posting to the forum afterwards, someone from the community offered a workaround).
I respect things can be complicated, but what would make that whole experience less frustrating would be either: A) If the reporting process was much faster (e.g. reporting directly within C3), or B) If more could be offered if a bug cannot be fixed, such as a suggested workaround or something.