Jumpthru behaviour works inconsistently if the object also has a platform behaviour (even if it is disabled). Some identical instances of the same object properly work as platforms while others let the character fall through. From what I observed, if there is a single instance, it usually doesn't work.
Attach a Capx
Description of Capx
It's simply an active platformer object with two Jumpthru platforms wich also have a disabled Platform behaviour.
Steps to Reproduce Bug
On start the characters lands on one of the platforms. Simply walk or jump to the other one. The bug is noticeable too by deleting one of the platforms.
The player object falls through the second platform, despite it being a direct copy of the first, functional one.
Construct 2 Version ID
(If somebody is curious about why in the world did I need such a behaviour combination, it was for making a crumbling platform. On timeout, it would disable collisions and activate its platform behavior to let gravity do its thing while fading out. I worked around the bug and simulated the gravity with a bullet behaviour using acceleration, which I guess is more optimal anyways.)