Having done my own tests with Resolve's Rec.709 option, I'd suggest not using it at all. It doesn't work well with ML raw files, the results are far off and you lose all the benefits of CDNG right away.
What I use to do is set color space to BMDFilm 4K and from there on use a LUT to get scopes into Rec.709 range. You can use the famous Hunter LUT (free) as well as
Vision Color's free VisionLOG in addition to "VisionLOG to REC.709/Neutral" or a workflow with the paid Cinelog LUTs. They all give slightly different images to start with but after adjusting the color temperature and exposure you'll always end up with better results than using Resolve's built in REC.709 feature. Plus you keep all the benefits of raw. The BMDFilm color space is basically just a starting point for the LUTs
and all options listed above are (according to their creators) designed with the behaviour of ML raw set to BMDFilm in mind.
If this doesn't work out, there might indeed just be something wrong with your files, I guess.
