I spent a lot of time trying to fix things and I documented it all here. https://forums.xamarin.com/discussion/comment/362759#Comment_362759
Unfortunately, the problem came right back.
Using Visual Studio for Windows 15.9.6(or the latest VS 2019 preview) and a connected MacBook running Visual Studio for Mac 7.7.4 and Xcode 10.1, the Visual Studio "Pair to Mac" functionality is improperly enumerating available devices.
From Xcode/Preferences/Components, if you download/install too many simulators, the corresponding list of devices that displays for the Visual Studio iOS projects, is unable to properly list or enumerate all these devices.
HOW TO REPRODUCE:
I am able to reproduce on multiple Macs, one from 2011, one from 2018, by simply adding all simulators including 10.3.1, 11.0,11.1,11.2,18.104.22.168,12.0. After adding these, go back to Visual Studio for Windows, and disconnect from the Mac, and reconnect. This will retry in enumerating all the devices.
I suspect this can be quickly reproduced as I was experiencing the issue across multiple macs/pcs.
Assuming you get the same results as me, you can then fix it by removing most simulators and keeping the one you want to test with.