I am having a strange audio delay.

The set up.ATEM Mini Pro with three cameras. Camera one has the mic attached and serves as the main audio. ATEM connected to computer via USB-C.

Laptop: Intel i7 9750@2.6GHx, 16GB and Nvidia graphics card

For a variety of reasons, we are sending the ATEM into vMix. I know we can stream live out of the ATEM, but for this gig it is not an option. We must go through the software.

All software (ATEM & vMix) are up to date.

We are sending the video from the ATEM at 1080 but broadcasting from the software at 720. The ATEM doesn't go any lower, but due to internet speeds, we need to lower to 720.

The issue.
When previewing the feed through the ATEM the audio synchs just fine with the video. This is as expected.

But through vMix (whether preview or live), the audio is significantly behind the video, even though the video and audio are coming from the same camera.

I always thought if the video/audio came from the same camera the signal would be synced? But not so in this case.

Here is the twist.
When we pull the audio through the ATEM input 1 (not through the camera), it syncs up fine with the video in vMix.

I am stumped. I would have thought the opposite.

In any case, any thoughts as to why the audio lags behind the video when brought in vMix? Is the encoder handling it differently? Hope you can help. Thanks.