Sounds odd… I have an Arc that doesn’t do that! You’re not switching your Arc off at night at all are you? Just trying to think what might be happening overnight… Curious. 
(Was the Arc new when you got hold of it?)

Yes brand new and sealed.

Not switching it off at all. Powered on and left plugged in. 

First of all, make sure you’ve run the latest system firmware update as there have recently been some Trueplay fixes.

Thanks. Ran the update. EQ settings saved overnight, but then reset again and trueplay off when I came home! 

If you are running TruePlay and then making manual adjustments you are in effect overriding TruePlay settings. Therefore, TruePlay is off.

Another possibility is this..,

Is the Arc plugged into a smart electrical switch that after the trigger component (i.e. TV) is turned off kills the power to other components which in your case would be the Arc?

Arc is connect to wall plug and via HDMI to the TV. I had a Beam before so it was a straight swap. My preferred settings reappear sporadically. For example open the app and Arc displays Trueplay  off and EQ set to zero, shut down the app, open again and Trueplay on and bass + 3 etc. 

I’d call your issue a “gremlin infestation” based upon what you are describing.  

Here’s what I suggest…

Run Trueplay and then run a diagnostic to establish a baseline and post the reference ID in this thread.

Follow whatever procedure you think makes Trueplay turn off. Run another diagnostic and post the reference ID as before.

Call Sonos tech support to discuss the findings 

Not sure where on the forum this arose, but I think it was about Trueplay and Arc. Apologies for being vague!

Someone had been watching the TV before Trueplay, so the source was TV when running Trueplay. This meant Trueplay didn’t complete properly.

So they played some music and then ran Trueplay, after which it ran fine.

Now, I know your Trueplay does complete for you - but it is not keeping its settings. SO maybe try:

• ...watching TV so that TV is the source, and then try Trueplay. See if that keeps the settings the next day.
• if not, then listen to some music so that TV is no longer the source, then try Trueplay. Again, see if that helps.

  It’s a stab in the dark, but you never know. I definitely read something like that!!

I haven’t had truplay turn off as far as I know but I have had the EQ and Sub settings reset to zero multiple times over the last week on my Arc. Got the arc 6 mths ago and was fine until this. No other Sonos product affected.