I just finished successfully recording on Minecraft Bedrock Edition 1.21.101 and wanted to record on Preview (for a bug report!)
I changed the recording source on OBS Studio as usual with Preview running and just get a white screen. I normally get Minecraft Preview pop up as I did with Bedrock. I recorded successfully with this version of OBS Studio on earlier versions of Preview without a problem - OBS hasn’t been updated recently. You can see in the screenshot in OBS that the recording source is set to the Preview window.
Can you try changing your Capture Method to “Windows 10 (1903 and up)” and see if it solves your problem?