[1273] Need to recompile solution before running unit tests

using R# 4.5 as well as the newest build #1273, when I run unit tests in 
debug mode and change code between the runs, I need to rebuild the whole 
solution on order to let the R# unit test runner "pick up" my code changes. 
If I just run the unit tests without recompiling, R# somehow doesn't notice 
that the code has changed and just skips the changed projects when building. 
This is also true if I switch "build settings" to "always build" in the unit 
test sessions window.

These are MSTest unit tests. If I use the  build-in Visual Studio test
runner, I do not have this problem.

What  could be the problem here?

Thanks for any help
Urs