It seems to be a persistent issue that some owners experience & others don't. I had problems with it a few months ago then it just stopped by itself. Recent updates seem to have triggered the problem again though.
You can stop the flashing LED by pressing & holding the MIC button on the remote for 3-4 seconds. However, as soon as you return to a home screen or trigger certain apps it may start again & you'll have to repeatedly disable it manually.
The Google App/Assistant seems to be a factor with my a3 despite disabling various features such as OK Google from any screen. After a Google update a few weeks ago the mic & gyro's on my a3 were constantly live. The remote wouldn't enter sleep mode at all & I was going through rechargeable batteries every 72 hours or less. (I uninstalled all updates for Google, clear caches & allowed it to auto update again. Since then my a3 won't sleep whenever my U9-H is on a home screen, as soon as I launch an app such as Kodi the remote goes to sleep as normal.)
Unfortunately, you may just have to put up with it until another update sorts it. Rechargeable batteries would be a good idea as well.
It disables the mic on the a3 & allows the a3 to sleep normally . Obviously, you will completely lose ALL voice input/control, so don't forget you changed this setting if you intend to use voice controls again in the future!!
Choose Auto or USB-Audio - SAGE AirMouse if you'd like to re-enable the mic on the a3 at a later time.