I’m not sure about being taxed, but amounts over 10,000 usd in value are more likely to come under investigation, I’m not suggesting anything untoward in anyone’s transactions, but they may be delayed due to investigation, this I was told years ago by the manager in my KBank branch.
I was told years ago from my Thai bank that any value over 10,000 USD is liable to a higher risk of investigation for money laundering, meaning legit transactions could be delayed, I’m not suggesting your payment has been delayed nor am I suggesting any illegality going on here but if you’ve made regular transfers above the 10k usd then maybe they’ve been monitoring your transactions and thus caused issues.
Usually they check, but I’ve been asked once when using a one way ticket if I’d planned on doing an extension once there, (which I did ) this following the check in desk checking through my passport pages and I guess them seeing mostly yearly extensions or yearly multi entry visas with plenty in out stamps, so you may get away with it but best have a dummy onward ticket just to be sure.