When selecting text with the keyboard (for example by holding down shift and using the arrow key, or with shift+home/end, etc) the selected text is not put into the primary X11 clipboard. As a result, it can't be pasted with a mouse middle-click.

This bug seems to have been around for a while. Not sure when this functionality regressed. I think at some point in the past it was working, but can't remember when it stopped working. In any event, on Linux, text selected with the keyboard is expected to be put into the primary clipboard. Every other application does so, including text editors like Kate/KWrite or web browsers like Firefox.