For red or blue belt inserter can fail placing item, even if it have space. Stack size was set to multiple of 2 o 3 respectively to ensure item is placeable in full tick.
I was able to pinpoint that curve after inserter block can "micro back up". I attaching 3 blueprints. One on straight line, works fine.
One have curve after witch is sideload, last item is unable to be placed. To be sure that sideload not backing up, i made third setup that have curve and unobstructed flow and last item is still unable to be placed.
Compresion unit tests
Fourth(most bottom) setup is with clock extended to 109. It works but it produces almost compressed belt.
Problem is not existent with yellow belt.