Hi - I have exactly the same setup (q95t plus Denon 3700h), and exactly the same problem, and after a lot frustration I seem to have found the source of the problem:
The auto-on for attached sound devices (receivers) behaves differently if the last Source you selected on the Samsung tv was the HDMI 3/e-ARC source (which you could well have selected to see your Denon settings). If instead you make sure that the last source selected is either another external source (attached to hdmi 1,2, or 4) OR if you're not using any external sources just select Samsung TV (That seems to count as a source too). Then assuming that you've got Anynet/CEC selected in your tv settings and of course ARC/remote control etc set in the Denon, it'll now turn off and on automatically as you expected.
From that point on it behaves as normal, but if you ever want to view your Denon settings on the tv screen by selecting HDMI 3 source, you just have to remember to select another source after.
Of course, the irony is that people trying to sort out this problem spend all their time with HDMI 3 / e-ARC selected to look at their receiver settings, to try to fix the problem :-D.
The tv always has the last source remembered. Apps you install on the Samsung don't count as sources in this respect. Only external sources or the Samsung TV app (and I guess terrestrial tv, although I don't use that myself).
I hope this helps.
