Not sure if this really qualifies as a bug or not, but I found a somewhat unexpected interaction of some mechanics. I've been making combined requester stations that accept both fluid and item deliveries in my testing map, which work very nicely (great place to use per-item request thresholds). I've also had some success with combined provider stations.
Also, now that the "1s of inactivity" condition is optional at provider stations, I realized that I could very easily request, say, a half or quarter wagon of fluid at a time; the train will simply pull away once the requested amount is filled, and unlike with inserters the pump doesn't need to be emptied. The specific use case I thought I'd try was to provide lube for the new AAI Loaders in production blocks.
In my test block, I made a very small lube buffer at the mixed requester station and requested 2k lube (the AAI Loaders use very little). The same station was requesting water and stone (for making wood in K2 greenhouses).
Stone and lube were being provided from the same station. What I expected was for a fluid train to pick up the lube, and a cargo train to pick up the stone. What happened instead was that the fluid train was assigned both requests; since it couldn't load the solid cargo, it of course got stuck.
I know that cargo trains can receive mixed loads from item providers (for example, if a mixed requester wants half a wagon of coal and half a wagon of stone, and both coal and stone are available at a mixed provider, then a train could be assigned to pick up both at once). It obviously doesn't work with fluid wagons though, since they can only have one fluid at a time.
Not really sure what a fix would be. The obvious workaround is to keep fluid providers separate from item providers, and to not make hybrid fluid/item providers at all (although hybrid requesters work great, as I mentioned). It's also not a problem if fluid request thresholds always fill the train.
Maybe trains with fluid wagons could be barred from handling multiple requests at once? Of course, I then imagine a scenario where someone wants to make a train with multiple fluid wagons that's intended to carry different fluids (like say, lube in the first wagon, petroleum in the second, etc). I always use 1-1 trains these days, so I never really think about trying to assign different items to different wagons on a single train.
I also tried setting up a fluid-only mixed provider (providing water and lube) and a fluid-only mixed requester (requesting 2k each of water and lube), and my 1-1 fluid trains were always sent separately for that, so I guess Cybersyn already knows to prevent that for fluids.
I also noticed that your example blueprint book does not contain any hybrid fluid/item providers, so I understand if it's intended not to combine them that way.
