I raise boer goats so maybe not a bunch of help here, but...
From what I know they come into heat during fall time, but I have heard the same also.
You are correct about the 18-21 days though. That applies to all breeds of goats.
I know that in MN the daylight keeps even boer goats from cycling 3 times in 2 years. So my boers are the same as most dairy breeds; only fall time heat cycles.
Hope this helps...
Patrick