Your confusion is much appreciated
This is not an easy one, as the source, in addition to the display aspect ratio of 2.75:1 also has a pixel aspect ratio of 2:1 - so, non-square Pixels.
Please, enable 'outline content' under preview and post a new screenshot.
If you want to have fun, read this first:
viewtopic.php?f=2&t=1426
As a starter, and repeating most of what Ioannis already said - you need to put your content into a DCI scope container with an aspect ratio of 2.39:1. As your source is 2.75:1, this is 'wider' than DCI scope, so you will see letterbox bars above and below the image. This is 'normal' and has been done for big budget releases as well (e.g. The Hateful 8, La La Land).
Another option would be to fill the scope container vertically - but you would have to crop image off both sides or distort the aspect ratio (vertical stretch). It's your decision, but, my personal choice clearly would be to show the full image letterboxed.
Your choice for a 4k DCI scope container is the right one based on the source resolution.
Would it be possible for you to upload a short snippet of this footage somewhere? One second with image borders visible on all sides would be enough. A screenshot is not sufficient (as it will lose the non-square pixel aspect).
Which version of DCP-o-matic are you using? In this case, and following the thread I linked to above, you may actually need to use DCP-o-matic 2.15.29 to do it properly. I did do 2.76 in 2.39 a couple of times before - but these were with square pixels, not anamorphic.
This is where the current stable version is doing wrong:
Bildschirmfoto 2019-11-07 um 23.08.33.png
When doing 'no stretch' it should scale up and keep the display aspect ratio of 2.75:1, not the source container aspect ratio of 1.37:1. That's the reason your content stays compressed, and is not expanded. You're lucky we just had that thread I quoted, Carl has only just uploaded a test version that should fix this behaviour. It should scale the 2450/1782 to 4096/1490 and pad 113 pixels on top and bottom with black in order to fill the 4096/1716 4k scope container.
- Carsten