My Switch would get stuck in TV output mode whenever I undocked it. The screen would just stay black like it thought it was still docked, but I could still hear everything and at the right angle I could see the back light just wasn’t activating.
My setup: Switch → dock → HDMI into soundbar (Dolby atmos) → soundbar into TV (old LG). I used a cheap third party controller which was both the solution AND maybe the problem, though it could have just been the switch itself.
Controller: Look at this junk
1. I had my Switch displayed on the TV normally while docked
2. I turned the TV off but left the soundbar on (I could still hear my game).
3. I paired this controller via usb while docked. Then I unplugged the controller.
4. Once I had the controller unplugged and working (tv still off) I undocked the switch and bam!
It seems for my specific setup that the controller is messing with the audio/visual chain. This controller goes to sleep really quick when inactive and I noticed when the switch is undocked, the switch goes to sleep mode at the same time as the controller. So basically I’m blaming the controller.
Hopefully you have a similar setup and this helps you get creative and troubleshoot for yourself!