Differences between PX4 and APM telemetry implementations

I’m trying to figure the differences in firmware between that of PX4 and Ardupilot. I have a video system that seems to only work with PX4 firmware on my pixhawk(I have a 4, a 4 mini, and a couple 2.4.8 pixhawks). There must be something different with how mavlink is implemented on PX4 and ardupilot firmware.