parts between opposing pistons disappear on reloading

1) switch to the command pod
2) move slider 1 to 100%, then to 0%, then to 50%
3) fly the plane over the mountains to the east, then return to the runway (touch down at 100 m/s)
4) the block should have disappeared

Edit: it's not just opposing pistons, it happens on other pistons too. Maybe just horizontal because I was unable to replicate with vertical pistons.