I have an Alexa group with two Treat Life wall light switches and no other devices. When I “Turn on Theater Walls” it turns on the projector and not the receiver. When I say “Turn off theater wall”, it kills the projector and the receiver if on. I am having an issue with the Fire Blaster so no IR repeating as of now. If I power on the Fire TV remote, It turns on projector and receiver. When I push off, it only powers down the receiver.
Epson 5050UB
Yamaha TSR 700
That sounds very frustrating! 
Just to check…this forum is so customers can help other customers with the Samsung SmartThings ™ home automation system.
Are you using a routine in the SmartThings app to control the devices? If so, can you post a screenshot of it?
If you are not using Samsung SmartThings ™ at all, you will probably get more help in an Alexa forum or by contacting Alexa support. 
Agree that this didn’t seem to be a Samsung SmartThings issue.
However, I had similar issues trying to manage my Receiver (Sony in my case) separately by itself so I no longer use any SmartThings or Alexa Routines to turn On/Off the Receiver power (one less SmartThings Alexa virtual switch).
Since both my HDTV and Receiver have built-in HDMI-CEC support (aka HDMI control), I set the HDMI-CEC features on both devices so that the Receiver follows (mirrors) the HDTV power. I turn On/Off the HDTV, and it automatically turns the Receiver On/Off with it via HDMI-CEC.
If your devices support HDMI control features it might work for you.