Copied text from rich text editors seems to be stored in the clipboard as a bitmap in X11
Lately, under an X11 session, copy and pasting has been doing weird things. It would appear that copied text sometimes gets stored in the clipboard as a bitmap. This has been experienced in various rich text editors such as LibreOffice and Geany/Geary.
Please refer to the following downstream report for more detail: https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/1852183