I can't replicate your case exactly, but....The pc is a thinkpad x280
uname -r: 5.2.9-200.fc30.x86_64
video adapter is usb-c to vga and cable is vga to the monitor's vga input
external monitor is DELL P2214H, 1920x1080
$ loginctl show-session 2 -p Type
Type=wayland
$ echo $WAYLAND_DISPLAY
wayland-0
lspci | grep VGA
VGA compatible controller: Intel Corporation UHD Graphics 620 (rev 07)
bootup is just what you want/expect, i.e. both monitors up and useable
I also have 5.2.8 & 5.2.5 kernels. Results are the same with them
I intend to get a usb-c to hdmi video adapter in the next day or two.
Let me know if it would be useful to repeat these using that.
HTH