Deliver 2x cement to concrete plant, craft concrete blocks, press f to load and 1x concrete blocks appears in truck, press f again and nothing else appears in truck. Press f again and the 1st concrete blocks vanishes from truck but now there's no concrete blocks in store either! Back to warehouse, more cement.... Back to cement plant, craft blocks, load in truck and everything disappears and I end up with nothing again! Back to warehouse, more cement! Back to cement plant, craft concrete blocks! Only this time I only load 1x concrete blocks on truck, close menu and blocks are still there, great. Deliver to garage. Now back to cement plant to load 2nd concrete blocks! Press f but instead of what's shown (concrete blocks) concrete slabs appear? Press f to unload concrete slabs which disappear along with the concrete blocks! Now I could do this all day but eventually I'm gonna run out of cement!
Well known bug:
I'm gonna run out of cement!
A work around is to craft one more than you need. So bring enough cement and craft three concrete blocks, then load concrete blocks FFF and you'l get two concrete blocks and the next cargo available ..