Issue capturing audio output from Ableton Live through my interface

[TwelveClouds]

Hi all, thanks for your time.
I have two audio captures setup in my stream: INPUT capture and OUTPUT capture. Both are from my interface (Behringer UMC 1820 OR Steinberg UR-RT2 - I have both and am having the same issue with both which is how I know it's not an interface issue).
I can capture the INPUT from my interface, no problem. I have a microphone on input 1 and I get good signal from it.
I can capture the OUTPUT from my interface when I'm playing music on Spotify or watching a video on YouTube, no problem.
So I have INPUT and OUTPUT and they're both working, *except* when I'm creating sound inside of Ableton Live. In Ableton, my audio is going through the interface, so it should be captured by OBS Audio Output Capture, right?
Am I missing something simple here?

 

[TwelveClouds]

For the time being I guess I'm lucky that I do have two interfaces and a small mixer because the workaround I found is this:
Using the Steinberg Interface for OBS to capture audio input.
Using the Behringer Interface with Ableton.
Sending audio from Behringer interface into a mixer + my streaming mic and sending that audio into the Steinberg.
This isn't really ideal but it definitely works fine. I'd like to just be able to use one interface though.