DCP can't export 1:1 video resolution correctly

Hi, i have a problem with DCP export in Davinci Resolve Studio 20.
The project we've been working on has a resolution of 2160x2160 HD square, and when exporting to DCP with the settings I've provided below, our 1:1 aspect ratio short appears "centered" in the frame and doesn't take up all the space it should on 16:9 screens, for example, since it doesn't touch the top and bottom of the monitor, leaving a large black frame around the video. What are we doing wrong? We're recent filmmaking graduates, and this is the first time we've tried exporting to DCP. It's also worth mentioning that we exported it using more traditional methods, and the resolution doesn't have this issue; only with DCP.
Thank you in advance for your understanding and help.

Format: DCP
Codec: Kakadu JPEG 2000
Type: 2K DCI
NOT HDR
Yes Use interop packaging
Resolution: Timeline Settings (2160x2160) (This changes as soon as we change the "Type" setting above, showing the 2048x1080 DCI option, but most of the times we have tried to export it we maintain 2160x2160)
24 frames p/s
Mbit/s: 250
--------------
ADV SETTINGS
Pixel A-Ratio: Square
Data Levels: Auto
(Options under Data Burn)
YES Force Sizing to highest quality
All others NO
Enable flat pass: OFF
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COMP SETTINGS FOR DCP
Projector Aspect Ratio: Full
NO Use encode aspect ratio
Resolution: 2K

DCPs can only be one of three aspect ratios:
2.39:1 (scope)
1.85:1 (flat)
1.90:1 (full) - not used very much

each of these has defined 2K and 4K resolutions

1:1 is not a supported aspect ratio, so if you want to put 1:1 out in a DCP you will have to have it pillarboxed in one of those three aspect ratios.

also 1.78:1 (16x9) is not an aspect ratio for DCP so that is why it doesn't fill your 16x9 display. DCPS are for projection where that aspect ratio is not standard (again it can be used, just contained within the other ones with letterboxing)

since you have 2160 high, your best bet in my opinion would be 1.85:1 (flat) 4K which is 3996x2610. You will just have pillarboxing when viewed in theatre, which is normal. If you view it on a 16x9 display, you will also have letterboxing - just by the nature of DCP aspect ratios all being wider than that aspect ratio.

Hope that makes sense..

Why are you making a DCP in the first place?