I have been using the ACI always on sandbox for a while as I am learning how to use several tools to automate a number of tasks in the ACI.
Previously when I have used the sandbox it has carried out a reset maybe once a day or once every 2 days.
Over the past weekend it appears to be resetting itself every 15-20 mins or so which, although not being a major issue, can be a little difficult at times as your tests will fail while the environment is going through it's reset process.
I'm not sure if this is a fault or it is a change to the always on system and is working as designed.