MSBuild.exe is not closed after every build

For some time (few months) we encounter a issue with VS 2015 + Xamarin, with every iOS build MSBuild.exe is not closed and every time new instance is created, after 2-3 builds CPU (Intel i7) is at 100% used by 3-4 instances of MSBuild. Workaround is to manually kill the processes but how long will it take until it will be fixed?
The issue happens on 3 different machines in the team, we are using Windows 10, VS 2015, Xamarin latest stable release, real 2 Mac Mini.
Thank you.