I've set up a planet for production only, no extraction.
I set up the production chain, routed all produced materials, checked and double-checked that every is routed and going where it is needed. I bought enough P1 for a whole number of final product.
and here is what happens. sometimes when all materials are consumed, I have one less final product than expected. lets say I bought enough P1 for exactly 10 P4, yet at the end I have 9 P4 in the launchpad, and some factories will have some materials in them, but not enough for the last 10th P4. as if somehow some original materials just vanished!
what is happening? the very first time it happened, I thought I messed up, so I have been extra careful. but it happened a few times more already and losing the final product like that is not making me happy at all.

production on all factories starts almost at the same time - I deliver all material at the same time (if I just set up a new production line, I route all P1 from launch pads and press accept when all lines are finished) or within a few seconds (if the production chain is established and I just need to bring new materials from customs office to 2 launchpads), so there should be no bottleneck anywhere and products should nicely flow from one factory to another.

The only thing I could think to check is storage. If at any point your storage facility does not have sufficient space, anything attempting to be store simply vanish.

nothing goes to storage except the final P4 product

If you have multiple factories the raw materials could be split across multiple factories. That is to say if you have two factories making P2 items for example, one raw material can get sent to one factory and the other could be sent to the other. Thus you would have enough materials, but no single factory has all the required materials.

PI factories love to split the materials between them. make sure your P1 aren't spread 50% needed amount across your factories.

If nothing goes to storage on the way from P1 to P4, then something isn't quite synched up in your factories. One of them is producing something before the next factory in line is ready. Since it's not going to storage and the next factory is still occupied, its just lost. Everything should be output to a storage area, even if it is immediately used.

Or it's like the poster above mentions. If you are producing 7 end items using 6 factories, all 6 are going to try to pull in materials after they finish the previous run. That will split the input like he mentions.

Another thought, are you doing:
P1 -> factory -> storage -> factory (etc)

Or

P1 -> factory -> factory (etc)
?

the production on all factories start either simultaneously (if it is a setup for new P4 product, I can do the last routing from storage to P2 only when I imported P1 into storage, and I press submit after ALL routing is finished), or with a few seconds delay (if the routing is established, there is a short delay between importing materials to different launchpads). the production cycle is 1 hour, and the delay would have to be more than that in order for materials to get lost.