I have the exact same problem. I have a Roku 3 stick plugged to my TV and powered via USB coming from the TV. This has been working with no problem for the last 3 months (since I set it up).
However for the last week (probably since I tested the Miracast feature 2 weeks ago), the stick will need a complete set up every time I tried to use it. As soon as the TV is off, the stick will loose all its settings and requires me to go through:
Link to my account
Loading the channel
Basically, about 15-20 minutes set up making it useless. I went through the whole setup 2-3 times already, and checked that I'm on the latest build. But it is as if the stick doesnot keep the settings in its memory anymore... Very disapointed for something that I got only 3 months ago