After some time spent on building my transportation network I discovered a very annoying bug. Complete list of strange behaviors:
1. Some trains, that should carry goods are leaving supplier's siding empty (with payload set at 100% in schedule!) then going to random (!) point in schedule and reversing, then going back to the supplier.
2. Other trains take the cargo but aren't going to the factory's customer - they are also going to random point and reversing to supplier.
3. Some trains are reversing twice (!) doing the loop before reaching receiver's siding.
4. I've even seen a train, that couldn't leave it's cargo at receiver - it took the payload all way back to the supplier.
What's going on? If someone wants to see it I can upload the .sve and whole pakset to Dropbox.