Render keeps switching to Quicktime from MP4

I'm brand new to Davinci. I just created a video for YouTube and, following their guidelines, was trying to render it for YouTube. They recommend MP4 but whenever I set the choice to MP4, once the rendering begins, it defaults back to Quicktime. Does this matter and is there a way to get it to stay with MP4?

Thanks much.

Render to Cineform, then use FFMPEG to encode H264 with CRF 12-14 and preset slow.

At least if best quality on YT, manageable upload and encode time is what you are after.

Alternative to that with an Nvidia card is to use the Nvidia encoder within Resolve to encode H265 with best settings. Less quality, but faster encode an upload time.

Anything else is not attractive for one reason or another.