This is about the Amazon Echo dot 2nd generation please try this with your Echo dot and Echo devices and let me know the in the comment section. How to try this: Please turn off the lights in the room and press the mute button so that it will be more easy to find out. It will also happens on the normal wake light in the Echo dot but the red light is easy to find. Once the mute is turned on please place the torch light or any bright light on the top of the Echo dot so that you can see the ring light on the will automatically brightness and once the light glows bright remove the torch light form the top of the Echo dot so that with few seconds the ring light on the Echo dot will again dim's. I really don't know does Echo devices has the light sensor I check on the Amazon websites also no one mentioned this happening on the devices.
For more details I have made a video about this please check this:
Please try this and let me know in the comment section.