the problem here is attempting to change layouts while platforming behavior is active. everything except the player character is local, while the character's hitbox global, and each time he goes off screen he's moved to the other side of it before the layout change.
if it's done while jumping, everything works just fine, but if walking from one screen to another i gives pretty strange results, with the character often getting thrown high into the air, underground or into similar weird situations. sometimes it even causes the game to crash if the character's placed in some weird place on change.
run forward/backward and try changing screens to see. if you change screens while jumping everything will work right, but during running it'll cause that bug.
construct version 0.98.8
a quick reproduction of the bug
updated with construct classic, additional debug key
construct classic r1.2
the problem still happens.
i added a new debug key to the new example file, press f2 to instantly change layouts. you'll notice, the player object will often relocate to a random position, if you press it fast enough it'll be shot high into the air for no reason.