There's no picture (the screen is black) on Windows 10 v1803 without monitor attached.
This prevents remote controlling of (my) standalone hosts. (I've tried RealVNC and TeamViewer so far.)
The previous version (v1709) was fine.

Will it work if you attach a plug into the card to make it think it's connected to a monitor?

I use these to trick my computers into thinking they are plugged into a monitor.
https://www.amazon.com/CompuLab-fit-Headless-Display-Emulator/dp/B00FLZXGJ6/ref=sr_1_3?ie=UTF8&qid=1526600132&sr=8-3&keywords=CompuLab+Display+Emulator+%28fit-Headless%29

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I use AnyDesk for remote controlling my computers without a monitor attached and it sill works with Windows 10 v1803!

There's no picture (the screen is black) on Windows 10 v1803 without monitor attached.
This prevents remote controlling of (my) standalone hosts. (I've tried RealVNC and TeamViewer so far.)
The previous version (v1709) was fine.

I had the same problem on two systems where I have an an HDMI switch. Splashtop has a "mirror" driver which may solve your problem. When I installed it I noticed the property detail said "realvnc" or something other than splashtop. It may work.

My actual headless systems worked fine with a 5pack of hdmi dummy plugs for about $16 from china mining suppliers. Long wait and one has to watch the adv on ebay as they all show 4 or 5 units but the fine print may say you only get one.

The two systems on my HDMI switch are a movie server and an archive system and I have to momentarily connect the HDMI before either will work with VNC.