@Abraver2 I'm having almost exactly the same problem, and it's been happening for a month or more. There are a couple of differences. One, the light is connected to one HS210 and one mechanical 3-way switch. (I have exactly the same setup for another light which always works correctly.) Second, it doesn't matter how many times I tell it, it still flashes on, then goes off immediately. Also, this only happens when the command is picked up by a certain echo device, and seems to intermittently work ok; and it always works fine when other echo/echo dot devices respond to the command. So that made me think that perhaps that one echo device just needed to be restarted. I just did so, and for now it seems to be working correctly. Are you still experiencing the problem?

  @NotQuiteAncient I am! Still no resolution unfortunately. 

  @Abraver2 Just so you know, I am too. It comes and goes, with no obvious reason. Wish I could figure it out!