A likely cause is that one of the cards cannot keep up with the camera’s write performance, even though no error was shown. In a dual-card backup setup, if one card is slower or less compatible, you can end up with missing frames on one card while the other records them.

Based on the reported experience, the most likely fix is to replace the slower card with a faster, better-performing one. In your setup, the SD card is often the more likely bottleneck, but either card could be the issue.

To reduce the chance of it happening again:

• use two high-quality, fast cards from reputable lines
• avoid mixing a much faster card with a much slower one
• format both cards in-camera before important shoots
• test the exact card pair before critical work by shooting a long sequence and comparing files on both cards
• if the problem continues, retire the suspect card and replace it

For critical documentation work, verify both cards after testing any new media combination.