See https://en.wikipedia.org/w/index.php?title=User:99of9/sandbox3&oldid=645853719 for reproducible sample, including instructions for reproducing several related problems. This was reported as being reproducible in the latest Firefox on Windows.
Steps to reproduce this one:
- Open page containing text and a <ref>.
- Select the sentence.
- In the upper right corner of the browser window, there is an "Open menu" icon (three bars). Click it, and choose the "Copy" button from the top of the menu.
Result: The selected text disappears, and is not stored in the paste buffer.