Help reading alarm open collector using xiaomi contact sensor as dry contact sensor

I’m trying to connect an open collector output on my alarm (2GIG GC2) to a xiaomi aqara contact sensor I’ve modified to be a dry contact sensor (like this) in order to determine whether or not the alarm is armed. I’ve verified that the open collector is an open circuit when the alarm is disarmed, and switches to ground when it is armed, by using a multimeter to test for continuity. I’ve also verified that the aqara sensor reads as open when the two leads are disconnected, and closed when the leads are connected to each other. So, in theory I think I should be able to connect one lead to the open collector and the other lead to ground on the alarm. Then, when the alarm is disarmed it will read as open, and when it is armed it will read as closed. Here’s my problem though - with this setup the sensor always reads as closed. What’s even stranger, when I disconnect it, it reads open, but then as soon as I connect even one lead wire to either the open collector or ground, it immediately reads as closed. The sensor then continues to read as closed even if I disconnect the lead. The only way to get it to read as open again is to touch the wires to each other and then pull them apart again. Any idea what I could be doing wrong?