Bluetooth Headset suddenly stopped using google now

I bought an LG Tone Pro HBS 750 about a week ago, and it's been great. The phone button, on the left, activated a Google Now voice prompt, even when the phone was off. However, this morning, it suddenly started using the Voice Dialer. I've been Googling for the solution to this problem all day, but all the results I can find are people asking how to enable Google Now voice actions. All the answers require downloading some kind of app, which I don't want to do because it clearly worked before. I've tried:

• Resetting all the app defaults. The button doesn't (and never had) prompted for a default action app


• Disabling Google Dialer. This makes the phone app unavailable, but the voice dialer still works


• Force stopping Google App. This seems to be the app that runs the voice dialer, but stopping it doesn't help. It can't be disabled.


• Unpairing and repairing the headset


• Resetting the phone with the headset on


• Resetting the phone with the headset off

This is on a Nexus 5 running stock Android 5.1