Resolution 2732X 2048

Hi, I use a 15inch MBP,2019 on 10.15.4. When I enable 2732X2048 on the iPad Pro 12.9, 3rd gen, the iPad first runs it for a minute or so, but then reverts back to 1366X1024. This used to work, I previously used an older iPad and MBP, so it seems strange that it won’t work on newer ones. Am I doing something wrong?