SUMMARY
xdotool type is not able to enter text into a GTK/Gnome application running on KDE Plasma

STEPS TO REPRODUCE
1. Open Gedit
2. Run 3 commands in terminal
windowId=$(xdotool search --name 'Untitled');
echo "windowId is $windowId";
xdotool type --window "$windowId" --delay 100 foo

The above is a "foolproof" method, but I previously used a one-liner in Gnome:
xdotool search --name 'Untitled' windowfocus type --delay 50 foo

OBSERVED RESULT
Text cannot be entered into Gnome/GTK apps by xdotool type when running KDE Plasma

EXPECTED RESULT
xdotool type, types the text into the desired application as expected.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:
Ubuntu 18.04
Plasma: 5.12.7
KDE Framework: 5.44.0
Qt: 5.9.5
Kernel: 4.15.0-43-generic
OS: 64 bit

ADDITIONAL INFORMATION
xdotool version 3.20160805.1
libgtk-3-0:amd64       3.22.30-1ubuntu1
libgtk-3-bin           3.22.30-1ubuntu1
libgtk-3-common        3.22.30-1ubuntu1
libgtk-vnc-2.0-0:amd64 0.7.2-1
libgtk2.0-0:amd64      2.24.32-1ubuntu1
libgtk2.0-bin          2.24.32-1ubuntu1

Also not able to send text into Remmina, but GEdit is an easier test.