This is a cosmetical issue; for some reason while on the road, the cargo (in this case it's planks) just disappears and then later reappears.

Nothing specific is logged to the stdout.txt.

First I assumed the cargo disappears in curves and reappears on straight roads, but then I recognized it's not like that.

Then I observed it a little better and found this out:
as soon as the blinker is set the cargo reappears for half a second, then as soon as the blinker blinks the 2nd time the cargo disappears again.