Weird icons in rdp session on external monitor from Windows 10

Question

I have a brand new Lenovo X1 Carbon (2017), and my problem is that when I'm inside an rdp session some of the standard Windows icons in the session looks super-weird. I'm not using any form of DPI scaling (it's set to 100% on all screens). My external screen is a HDMI-connected 34", so it has higher resolution than my built-in 14" screen. When I'm using my external monitor the laptop is closed, so there's only one screen in use at a time.

Here's an example of what the "caret" icon (text cursor) looks like in the remote session:

Anyone seen this before? Caret icon in an rdp sessions is this small glob of dots. Same in royalts and rdcman. pic.twitter.com/pQRniMG4VW— Trond Hindenes (@trondhindenes) April 29, 2017

I'm seeing this problem in both the rdman (Remote Desktop Manager) client, and RoyalTS. Even weirder, I'm getting the same in Virtualbox running locally.

My Windows 10 client is running the latest "released" version of Windows 10, which I believe is the "creators update" - it reports build 15063.138.

This is slowly driving me crazy - any pointers deeply appreciated!

Update: I did some more research, and here's what's happening: If I boot my laptop with the lid closed and connected to my external monitor, I'm getting the weird/small/unbearable icons. If I at this point open the lid and start any of the "problem apps" such as rdcman on my laptops built-in display, the problem persists.

However, if I boot my laptop without the screen attached, and then start rdcman on the local display, everything is fine. At this point, I can attach the external monitor, close the lid, and everything is still good.