I have the following setup of conveyor splitters and mergers:
This is a five-way splitter, where I currently split an MK4 belt, transporting 450-480 items / minute. This enters the merger, where the next MK4 belt should transport 540-576 items / minute, which is not possible. And I think this is, where the bug originates.
Due to the inability to merge in this continuous loop, an algorithm detects that the conveyor splitters and mergers can stop distributing items to reduce lag. But this is not taking the other conveyor belt in account.
When I remove the connector between the two conveyor splittors (One of them with the outgoing conveyor belt with no items on it), the cycle starts again, but completely deletes 50% of the items on the conveyor belts. This can be observed, because the originally empty conveyor belt now transports <= 240 items, which easily can be transported with MK3 conveyor belts, which I observed to be true.
I was not able to test, if a MK5 conveyor belt, which can handle the 540-576 items/minute task, is able to fix the bug, but I expect that to be the case.
I am currently playing Early Access CL#102023.
Edit: This behavior is reproducable. I have the exact same setup for crystals right next to the one in the image, and the same bug happened there, too.