OK Moved you to the right spot. There do seem to be some issues with the backwards compatible version of BL2 on XB1(X) when it comes to disk-based DLC. I’m not sure what to advise for this tbh - just had a very similar problem reported. I can only assume it’s some sort of licensing issue with the way the XBox handles DRM for backwards compatible titles.
You can try dropping a line to MS support or opening a support ticket with GBX/2K.
Since you have the XB1X, though, you might want to keep an eye out for deals on the Handsome Collection - you’d be able to transfer all your existing saves and get the higher frame rate performance (as well as the 4K textures if you have a screen capable of displaying them.)