After finding this post for the Comet, I was able to successfully apply my custom cert via the KVM’s terminal. High level steps:
cd /etc/kvmd/user/ssl/
echo "[PASTE YOUR KEY]" > ./server.key
echo "[PASTE YOUR CRT]" > ./server.crt
reboot
There is clearly a bug in the web GUI import process as this is the exact same cert I’ve been trying to use since I initially reported this issue.