How exactly did you check the memory? Fake cards report a bigger size than they actually are. You'll only notice it after you write more than their actual size to them, as the exceeding data will be lost, often without any error messages if the fake card was done well.
If you can boot to RetroPie's command line, you can try to install the tool f3 and test the card with that:sudo apt install f3 # install the tool cd # change to home directory makedir test # create a test directory f3write test # write 1 GB sized test files until the card is full f3read test # read the files' test patterns rm -r test # remove the test directory
It will test the actual amount of data that can successfully be written to the card and read again.