As you can follow in a long thread at macrumors, the returned SD card slot in the 2021 MacBook Pro doesn’t seem to be very reliable.
Many users are reporting that the Mac is having problems regardless of the SD card manufacturer. Either it takes a long time for the card to be recognized, the Photos app hangs, or the entire Finder itself crashes. It does not seem to matter whether the card was formatted in the MacBook itself or in the camera used, for example.
However, many users who wanted to get closer to the problem reported that all problematic cards for the MacBook Pro work with an external USB-C card reader. So the cause probably lies with Apple.
Those who can still remember the older MacBook Pro with card readers should not be unfamiliar with such reports. Sometimes you had to push the card into the slot at a certain speed or the cards were mounted, but only read-only. The only surprising thing about this story is that six years after the last MacBook Pro with SD card slot, the same problems start all over again.