I'm not sure what is happening but I would suggest some steps to do.
1) First of all, make sure your computer has an updated antivirus. Sometimes you need to run additional malware software, like malwarebytes, and other anti-spyware. There are some free antivirus online. Bitdefender online, nod 32 online. Use several, not just one.
Sometimes there is malware that tries to get into removable devices.
Scan your computer. Don't scan the SD card.
2) Try to use the usb cord instead of using a ssd card reader.
If you installed the software that came with your camera, in general terms is safer to transfer the files that way, because it recognizes different associated files, like previews.
3) Although there are some data recovery software, if the videos are really important go to a specialized company to try to recover the videos, because you just have one chance to do it. That is why I won't recommend any software.
The data loss could be either by software or a physical failure of the card. In the case that the failure is just by software it can be on 2 parts, the video itself or just the table info that find the file inside the memory.
4) Make some tests with new cards, and see if the camera is not the one that has the problem.