It’s certainly not the way an advisory role should work, but I wouldn’t go so far as to call it a red flag. A lot of time a company uses an advisor in bursts, and in my case they knew that I was ready to walk away in part because I was too busy for other commitments. So for a while I thought they might just be giving me space in order to retain me. And, because I was super busy, I wasn’t constantly thinking about their not having communicated with me for a while.
So I agree that it’s a flag, but I wouldn’t go so far as to call it a red flag.