I've had this exact same problem with more than one card. Have a Samsung Galaxy 10 plus with an additional 512gb microsd card. When this card is used in an adapter and a sd card reader on any pc, all files work perfectly. When this card is inserted into my Samsung phone however, about 1/3 of the files don't display correctly, or can't be opened. Once again, this same card always works correctly in an external card reader. I've already used SDFORMAT to reformat more than one card in these same circumstances. Since it's happened with more than one microSD card, I'm pretty sure it's NOT the card itself.

Anyone else have similar problem in their 10+?