For instance, if I try to edit in kate, and use the down-arrow, this is converted to enter, but only if I use the dedicated arrow keys. Number keypad arrow keys are fine.

konsole is the same with vim. arrow keys do not work.

Left and right arrow do nothing, up arrow also does nothing. Page up, page down also does nothing.

This problem does not occur in gedit, mate-terminal, or firefox.

kde-baseapps-bin 4:15.12.3-0ubuntu1
xrdp 0.6.1-2ubuntu0.3
vnc4server 4.1.1+xorg4.3.0-37.3ubuntu2
mate-session-manager 1.12.2-1ubuntu0.1

Ubuntu 16.04.6 LTS