It would not surprise me at all that the majority of CMs at DLR who work on FP rides are unaware that tickets scanned at the opposite but not in their park can still be used for FP acquisition. It is kind of an advanced concept that FP gurus know about and that is about it.
Disney does not tend to school their CMs on all esoterica of possibilities. Rather they try to standardize the CMs on the same info for the greatest possibility of accuracy to guests.
Please don't take my post here as denying your experience. The practice of scanning tickets at one park and then using them at the opposite park for FP acquisition has been in force since at least 2005. I have personally done it in both directions at the two DLR parks many times - probably close to 20 times - from 2005-2012.
If the tickets are parkhopper tickets (do you know for sure the irate guests had PH tickets?) and
if the tickets have been scanned at the opposite park on that same day then without fail for the last seven years this has worked for many, many people.
Your experience differs from this and so it is expected that folks would be skeptical - not of the experience, but whether all the facts were properly accounted for.
With that said, if DLR is changing things up then the only we will find out short of them announcing it (and they never announced the dual park FP policy in the first place) is for people like you to report it. And then others report it. And then finally someone authoritative (maybe an editor of one of the online forums or maybe RideMax or TouringPlans confirms it). If they really are doing this policy randomly on different days that seems inconsistent with how Disney operates but it would make it very hard to confirm.
The other thing about this that is hard for me to understand is that with both parks opening at the same time on most days and FPs for RSR running out so early (by 9AM often reported) it is hard to see how folks would first enter DL with their group, then leave DL right away through an exit and get into DCA, get in the RSR FP line (which will be monster by that time - say 10 minutes after the park opens) and then actually getting a FP. Can it be done? Sure, I think I could do it. Would it be hard under today's RSR FP demand? I think it would be except for all the most savvy of DLR experts. And on some days probably not even then.
I guess time will tell. I hope I did not make you feel disbelieved. I want to try to add some historical perspective and a little outside current reality perspective to the whole thing. I also hope to not discourage you or others from posting experiences like this because it that happens we will not learn of new trends at the parks - which is why many of us come to forums like this.