Ive never turned mine off before changing the cards.
Sounds like a card reader or formatting issue
Yup, I have 8 Wild Game cameras and have never turned one off to remove the card, never been a problem.
But I have had some that for whatever reason will turn themselves off when I remove the card. So when I put cards back in, I check that they are powered up, or no pictures.
"Man is still a hunter, still a simple searcher after meat..." Robert C. Ruark