The Sandisk should be OK if it's marked "A1" which is their newer production, their older red/gray cards were troublesome for dashcams. Staples is a trustworthy source so it shouldn't be a fake card but you could verify that by using the "h2testw" freeware doing a full overwrite. The Lexar isn't likely to be usable here; they sold off their memory division a couple years ago and something changed with their card quality after that. Before then they were one of the best cards for dashcams but now everyone who tries Lexar has problems with them. It might work OK in your Olympus as it doesn't need the high write speeds that dashcams require.
So if your Sandisk tests OK I'm not sure what could be happening. Maybe someone else here knows the cam better than me and can jump in to help.
For my LK-7900, the in-cam format is actually just a delete function, a quick format. So if the SD card file-system is corrupted or scrambled, it doesn't fix anything and strange stuff still happens.
This might be what you are experiencing with your LK-7950.
To fix it, I put the SD card in my PC, check if the card is OK out of curiosity (right click, Properties->Tools tab->Error-Checking, check now button) to see if the file-system is OK.
Copy config.bin to a safe place and manually format the SD card (64K, FAT32 (for 7900)- not Quick Format), takes a long time 15minutes/64GB.
Then copy the config.bin file back over, and it's all good. The cam creates the usual folders.
Sometimes I still get really old files appearing, I've erased/formatted the card and looking at it can see stuff from a couple months ago, which is strange. I usually in-cam format every time I fill up with gasoline.
Also make sure you have the latest firmware in the cam, Lukas did fix SDcard firmware-related issues years ago.
Beyond this, the SD card might be worn out.