Default Does ART-mode minimize phone app a few minutes in?

Just wondering, this seems to be something I notice quite often and its annoying because I either have to unlock my phone, or the phone app is minimized and I have to pull the notification panel down to go back into it, or hit phone app icon (if i remember to do this).

Is there a work around?

I end up also missing call prompts on automated options
Nexus 5 16GB Black