Memory card is wiped after importing to Lightroom

This is a known issue with XQD, CFast, and CFExpress card readers. The operating system shows these as external hard disks instead of removable media and Lightroom Classic will not realize it is a memory card and offer you to "Move" images which it won't do if it realizes you are importing from a memory card. There is a feature request here somewhere to change this behavior if Lightroom notices a DCIM folder but it has never been implemented by Adobe.

It started when I purchased a Lexar C Fast memory card for my Canon EOS DX MkII.

Check at the top center of the import dialog that you do not use MOVE. That should not be available, but it's possible that the CFast card is seen as an external drive, rather than a memory card.

Hmm, deleted? Are you seeing they are gone via Finder? or via LrC?

In your Import methods, is ADD or MOVE  available. When importing from camera or card, they should not be available. If they are, a problem exists.

In the LrC Import options, have you selected to Eject after Import, to automatically eject the card? That saves you from damage when actually removing the card if the computer is still communicating with it.

MACOS is especially finicky about that

Please describe exactly how you import. If you use Apple Image Capture, then this could happen because this is an option in Image Capture.

That is not a normal Lightroom procedure, and should not be happening. It is recommended that images on the card should be deleted in the camera, and that the card should be formatted in the camera. Does this happen every time? What operating system are you using? Also, what exact version of Lightroom are you using?