I have noticed that Painter 2021 under macOS Big Sur (recently updated to 11.2) does not detect my thunderbolt 3 eGPU Radeon 5700XT in the Painter performance settings. It seems it does not even detect the Radeon 460 built into my 2016 MacBook Pro either. The option to use the GPU is greyed out in the Performance section of Painter preferences. I have even tried setting Painter to prefer using the eGPU via the Get Info method. No luck. I hope Corel fixes this issue soon as it's a big performance hit while working in Painter.
Just an update… After back-and-forth emails with Corel support I eventually had to completely uninstall Painter 2021 and then reinstall it to get it to recognize my eGPU.