render in place to mp4 not possible

Hey friends - I've noticed throughout many of these betas, when trying to use "render in place" on the edit page (great function by the way, thank you!) that it is not possible for me to actually render to an mp4 file. it says that I have selected h264 mp4 as what I want, but then it only will render to mov files. Is this a glitch?
I'm always trying to use this with native h264 mp4 files stacked on my timeline - so I'm always trying to select multiple files to render in place. Never allowed to render in place to mp4 though

I'm seeing the same here.

Additionally, the DNxHR exports to MXF OP1a files don't play outside of Resolve using PotPlayer, which plays pretty much everything.

They are probably doing this to be sure about TC, which is flaky in mp4. The codec will be the same, so if you need mp4 wrappers afterwards, re-wrap with any of the free utilities. It's fast and lossless.
Of course, it's still a bug if they are offering the option in the menu.

TC, which is flaky in mp4.

Given the sheer number of cameras that do so, I'm skeptical.

Let me reword it: not flaky, but not cleanly standardised.

Of course, you can blame BM, since some NLEs read it just fine, some not. But it depends on the camera which was the source.