I am making an assumption that this is a well know issue between the interaction of the platform and the jump-thru behaviors, since I have seen posts about it dating back at least 10 years, so i'll try to make this post concise.
If a platform object has an event that tells it to always fall through a jump-thru, it will still always stop its vertical velocity for a frame while falling through an object with the jump-thru behavior.
If the jump-thru is a few pixels thin or thick doesn't seem to matter.
If the timescale of the project is set to 0.1 it doesn't seem to happen, which makes it seem like a bug rather than an oversight.
I only own C2, but I found the issue still persists in C3 after testing it in the free trial.
It's extremely easy to replicate and is consistent.
Because of the versatility and utility of the platform behavior, this should be remedied.
If it is a problem within the hard-coding of the platform behavior in relation to the jump-thru behavior then the addition of an action to enable/disable the interaction between the two would seem like an obvious fix to this long running problem and help maintain the said usability of the platform behavior.
In short, an addition of a means to turn the interaction between the platform and jump-thru on/off in the platform behavior properties, as well as during runtime.