While installing a new FlexIO addition AB fessed up to an undocumented "gotcha".

If you mix analog inputs and outputs in the same FlexIO "rack", do NOT put one of the analog cards in the first slot right next to the network interface. Apparently there is a timing issue where this won't work. You can either use a discrete IO module or leave it open. This was with an ASB (DH+) module as the interface.

Did the person you spoke with say if this is a "won't ever work", "might not work", or "might not work intermittently" sort of problem ?

Did they mention if it's specific to a certain Series or firmware revision of the 1794-ASB Remote I/O adapter ?

I've been on the inside of 1794-ASB protocol and function diagnostics in the past and there's usually more than one thing that contributes to a nonfunctional feature, or makes a malfunction more likely.

Two documented 1794-ASB issues regarding block transfers (which analog modules use) are documented in Knowbase Answer ID's 31314 and 19520. Do either of these sound like what was described to you ?

This is a issue with 1794-IF4I and OF4I modules that apparently uncovered upon release of newer ASB modules.
IF4I and OF4I rev G firmware fixes this issue

It sounds as if this is a blanket problem. We knew we had a problem because we were installing a new rack of Flex IO on an old PLC 5/20 (now upgraded to 5/40E due to blowing the IO memory space for the new additions). The new rack is an ASB module, an IF4I, an OF4I, two 16 input, and two 16 output cards (forgot the numbers). The layout was the two analog cards first, then the 4 digital IO cards. On EVERY Flex IO rack, we were fine until we plugged in BOTH analog cards simultaneously. That's when the rack went down. Since a sister plant already implemented the same project previously with basically the same IO arrangement, this was a surprise. We even tried disconnecting wiring in case it's a grounding issue.

AB tech reps started out with the reboot everything approach. Then wanted to set up another PLC with nothing else on it to verify that it was a software issue or isolate one of the existing IO cards.

But before going through most of that, they said that there's a problem with IF4I, OF4I, and IF2XOF2I. It's not documented yet and that the tech note would be coming out soon. Essentially if you have mixed (output and input) cards at least with an ASB module (I don't have full information so I can't extrapolate to what other situations would occur), The limitation is that you can't put one of the above 3 cards in the first slot next to the adapter. Anywhere else is OK. And you can put a discrete IO card in that slot. Just not one of those 3 analog IO cards.

I recall a similiar situation four years ago where you had to place certain analog inputs at the end of a flex io rack. Can't recall the specifics.