We saw 1 whale shark in my December 22 live aboard, well I missed it :-(
I started diving periodically in Thailand (and Burma) in 1988. I moved here in 2005 and have dived regularly since then.
I didn't see my first Whale Shark until October of 2012. That was at Hin Reua, Koh Kra. I've seen about a dozen since then. The last was in October of 2020 at Losin (see image below).
I have no explanation for this other than that I've heard several dive guides say that the twenty teens were good years for Mantas and Whale Sharks. I have no idea if that's true or not.