Always on Display setting turns itself off on iPhone 15 Pro Max

To set the scene:

I bought an iPhone 15 Pro Max at launch (I'm coming from an iPhone 13PM so never had this feature before so can't speak to previous issues or iOS versions).

I set this phone up as a fresh install without transferring a backup so there should be no settings or other old cludge coming across from prior installs.

I did log into my iCloud account.

I am running iOS 17.3 (current as at 8th Feb 2024) and have had the same issue since at least iOS 17.2.1 which is when I noticed it (but I don't actually know if it was an issue prior).

I have no custom shortcuts, routines or workflows setup. In fact, I barely use any features of the phone really.

I do have the Do Not Disturb (DND) activated on a schedule for when I sleep. This is something I have had since it was first implements way back when. I don't recall actually turning it on with this iPhone so I have to assume it came across when I logged into my iCloud account. I can Focus has an optoins to "Share Across Devices" so I assume this is it. More on this later.

I go into Settings > Display & Brightness > Always on Display then set all settings to On (Always on Display, Show Wallpaper and Show Notifications).

The phones use is very predictable, home on a magsafe charger everynight without fail (even in the rare instances I travel I use the same charger). I never physically plug the phone in to charge or transfer photos.

Life is good for a couple of days then my screen isn't always on, I go back into that setting and it's turned off. Turn it back on and the cycle continues. It typically lasts a few days, but could be as short as a single day. There doesn't appear to be any predictability to when/why this happens.

I had read some vague thoughts that it could be related to DND so tried manually turning in on and off a few times via Siri.

My phone was locked and sitting on my desk, Siri heard me, did its thing and the screen went black.

I again yelled at the air and asked Siri to turn off DND, again it did its thing and my Always On Display is back on. So it doesn't appear that this is the issue (or at the least the single issue).

It's also hard to search for similar problems as peple talk about the Always on Display turning off not staying turned on when the setting is on, not about the setting actually getting turned off.

Does anyone else seen this behaviour or have and suggestions?