I just wanted to confirm, that I was having the exact same issue between two machines running Windows 10 April update 1803.
Upon reading this forum post, I tested a few different scaling configurations. I found that so long as the server display that connects to the client display is scaled to 100% then it works.
In the attached photo the 4K displays are connected to the server, the 3K display is a laptop acting as a client. So long as the server display with the * is set to 100% scaling, then Synergy works normally, however if it is anything more than 100% then the mouse switches to