Hey C2 mates :)
I'm starting with my first test of C2 by making a runner game and I had the same problem even when I used a box sprite (not pinning it to the animations but setting the position in every tick,) but this problem fixed itself when I used only short solids with multiple animation frames. I'll explain.
The reason this bug was happening (I guess it's a bug, unless I missed something else, 'cause I've tried really everything to fix the problem), from what I understood, is that if I used a very wide solid that spans the width of the first screen or is wider than it (either tailed background solid or sprite solid) the player box has problem with that, disappears, or is inconsistent in movement etc, but once there were more objects on the screen that use all the three different animation frames of the sprite animation strip, the problem disappeared.
So if you ever still have this problem, try to use a sprite solid as a ground for the game with at least 3 different frames of possible animations (for example leftern edge, middle, and rightern edge), and place three of them in the first, hmm, window of the game (I mean the first screen, not the whole level, or maybe just go for it and use them outside the first screen in the layout) see if it solves your problem?