Even though it is a normal press of the OEM power button, Apple has figured out a way to not know how to launch art mode. Only turns completely off. Are you kidding me? Lol

Can’t tun CEC off cause there’s a Sonos Amp running off of it.

Apple Controls the TV Via CEC. Its not emitting and off IR Code.... CEC protocal does not have provisions for "Art Mode"

I guess Apple can’t just let their remote send IR because that would be too much like the normal way of doing things!

or it saves them pennies for something most people won't need

True. Save a few Pennie’s and charge extra for it!

Is there another box like Apple (Roku?) that can tell your Samsung to go to Art mode?
Not sure how this is an Apple issue, or why you would expect your Apple TV or it's remote to specifically have this control?
I thought if Art mode was active, it senses (motion) and will display the ART, so by turning off your Apple TV and still being in the room one would think the TV should default to Art mode. I will have to try this tonight.

I don’t believe the problem occurs with Roku and maybe Amazon fire, not sure, but their OEM remotes operate the television IR while Apple uses CEC through HDMI. If you are using a Roku remote and press the power button a short press of the TVs power button is emulated resulting in art mode. If you press and hold the power button it turns completely off just like the Frame TV remote does. With CEC control the ONLY option is off. I believe Apple is saving money by not putting an infrared emitter in their remote. Then they charge extra for their product lol.

This seems like a good Raspberry Pi project.

The new AppleTV remotes (Siri remote second generation) still have an IR transmitter, so the statements above about "penny pinching" or the remote "not having IR" are inaccurate.

As OP noticed, the ATV remote will default to CEC control if CEC is enabled on the TV.

Personally I think that Samsung should put the control over Art Mode in their TV settings (eg a "Power Off turns on Art Mode" setting) rather than stacking codes on the same button of the factory remote. Using it with any control system similarly requires switching between custom TV control profiles (drivers, modules) that substitutes the Art Mode commands for the Power Off ones, or vice versa. As it is I have to find out at install if the client wants Art Mode instead of Off, or the other way around; and then they're locked into that decision until a programming change is made.