I've got an early 2011 13" MBP running OS X 10.10.4 PB, but this issue has been happening on literally every version of Yosemite, stable and public beta, since the initial PB release. You know how if an app has no active windows but some windows minimized, clicking on that app's Dock icon will open a minimized window? Well, that just doesn't happen on Activity Monitor.
I've found no other apps, Apple or third-party, that have this glitch, but clicking on Activity Monitor's Dock icon will change the menu bar to Activity Monitor but leave the app's window in the Dock. How can I fix this?