I was offline with a local wifi network but no internet connection. I recorded several voice messages in telegram desktop, and they displayed as stuck uploading. When I returned to internet (on a different wifi network), telegram remained stuck. So I did the obvious thing and restarted the program. I have done so before in this situation and the messages have been uploaded successfully. However, in this case, the messages did not upload, and vanished from my own telegram feed.
Looking in ~/.local/share/TelegramDesktop/tdata, I found files that appear to be the voice messages. The timestamps and sizes line up. But the files are encrypted. And it seems that telegram has forgotten they exist, so won't send them.
This is the expected behavior. The card has been turned into a suggestion card.