Nexus 5 owner here.
Basically, I encrypted my phone a while back while on Dalvik Runtime and recently wanted to give ART a try again (to check app compatibility).
The problem is that when I set ART as the default runtime and reboot, it doesn't try to apply to change and reverts back to Dalvik. [it worked fine prior to encryption]. Is this a known/common occurrence (search engines didn't give me much)?