well this is frustrating.
tl:dr = With no pull-up resistor on the fault line, and in normal non-faulted state, the ACS711 is outputting 4.4V on the unloaded fault line which is supposed to be an open-drain with an internal 240K pulldown.
History:
I've successfully used these ACS711 series hall effect current sensors in a bunch of designs for a number of years, both personal projects and production stuff. They have their issues (like easily picking up adjacent fields) but do great for general current sensing. And they are cheap! I've even used the Fault line before (but always like a normal person, with a pull up).
Situation:
ACS711KEXLT-15AB-T is the version at issue here (but all from family probably apply). Powered from 5V. Confirmed the supply is good. Chip looks happy, with actual current output sitting nominally at about 2.5V and moving correctly with changes in actual current. Legit parts sourced from Digikey.
https://www.allegromicro.com/~/media/Files/Datasheets/ACS711-Datasheet.ashxI'm not sure what the deal is here. The datasheet clearly shows the fault output as an open drain with internal 240K pulldown.

When I measure the unloaded fault line voltage, it shows a solid 4.4V. Scope shows no appreciable ripple.
Shorting the fault line to ground results in the voltage collapsing but still sourcing about 200uA.
Another strange data point is that with a 7K load, it can still source about 150uA and the voltage holds up to about 1V. That sounds like a lot for some sort of internal coupling or other parasitic.
If I force the ACS711 into an actual over-current fault condition then the fault line pulls hard to 0V as specified. No issue there.
I cut the trace so the fault output line is disconnected from the rest of the board, so it's not something backfeeding it. I also replaced the chip (twice) to make sure I didn't just break something. Again, no pullup is installed here. There isn't anything on the board going on close by either (nothing switching, or high current). Total supply draw for this whole board is 13mA during all this testing.
Does this make any sense to anyone?