Imported MOV files not transparent in Keynote

I am trying to enhance presentations in Keynote by importing video animations as MOV files with active Alpha channel, from CreateStudio, but they are shown with a black background on a M3 Pro MacBook Pro, but with transparent background on an Intel based iMac (Retina 4K, 21.5-inch, 2017). Both use the same OS version (15.1, 24B83). Why?