>A huge red flag.
It won't be a red flag for people who often see auto-complete not working for legitimate websites. The usual cause is legitimate websites not working instead of actual phishing attempts.
This unintended behavior of password managers changes the Bayesian probabilities in the mind such that username/password fields that remain unfilled becomes normal and expected. It inadvertently trains sophisticated people to lower their guard. I wrote more on how this happens to really smart technical people: https://news.ycombinator.com/item?id=45179643
>So are there mobile password managers that don't even check the URL? I dunno how that works...
Strongbox pw manager on iOS by default doesn't autofill. You have to go settings to specifically enable that feature. If you don't, it's copy&paste.