I'm curious if irrops override some of the automated checks?

Last week, I had a trip (IAD-IAH-PHX) where the WX in IAH were not great. I took advantage to initially rebook for the following day, but as I had booked 2 one ways (as I usually do), I didn't want to touch my return trip right away in case it had irrops that resulted in more flexibility. This resulted in me having a PHX-LAX-IAD (the original return) and IAD-IAH-PHX itins that would have otherwise been impossible due to the redeye arriving close to (maybe even after) my departure the next morning. I've had UA issue the "impossible flights" notice before when I had a 4 hour gap between flights (somewhat more complicated as the 4 hours involved 2 different airports, both arriving into IAD, but quite technically possible to still do), so not having received the notice here was surprising. I did go ahead and move the return back a day a few hours later to avoid the risk of an automated cancellation, so it's possible the system just hadn't swept and caught it yet--but IME, it is usually within a matter of minutes to an hour or two at most before UA sends the notice.