Well this is getting confusing. One day it doesn't work, next day using the same file and focus pixel map file -- it works:

What is also weird was that I was getting corrupted exported dng files then I launched MLV App this morning but after relaunching the app later in the day it worked fine.
I am using Resolve 15. Just checked. It opens and desqueezes a 1x3 Dual ISO files properly in the way I explained above.
I'm on Resolve studio 16 public beta. Yeah, I know--show off. I got a complimentary dongle from Blackmagic because I was testing out their products on a consulting gig.
Is this your explanation?
In the Sizing window, just type 3 for the "Width" and 1 for the "Height" setting.
Where is the Sizing window you are talking about? Please show us.
What I did find is that a clip can be resized in the timeline using the Transform option but simply typing in 3 for the width (x) and 3 for the height (y) doesn't work because it extends outside of the frame. Seems like it is more complicated than that. I just did this freehand until it looked about right. Obviously there is a better way but it requires doing some math:

Not of much use though since Resolve does not convert Dual ISO files.
What workflow are you using? You need to convert Dual ISO in MLV App
before exporting the cdng files.
Well, skipping cdng tags is nothing new. Even adobe skips this tag in premiere.
True. Looks like the only way to get the right aspect ratio in Premiere is to do a dynamic link from After Effects. Either that or use a motion effect the same way as transform in Resolve.
By the way, both Resolve and Premiere has the option of changing the pixel aspect ratio but they are fixed options and none of them work with 1x3 sampling:

In Resolve you can apparently resize on import but I couldn't get it to work with cdng files:
