You are correct, the MX300 does support H323 duo video, so it should work. Is this a Gen1 or Gen2 MX300, and what is the other endpoints connected in the call?
Where it says "Content tx" for the MX300, what happens if you disable and then enable the content stream going to the endpoint?
How do you have the content channel configured for the conference, and what happens if you set it to either Passthrough or Hybrid? Refer to the MCU Help Guide on pg 53 for information regarding the content channel modes.