Perhaps the most critical part of the post is this:
I had purchased a one-way ticket of Bangkok Airways Flight PG 905 and a one-way return ticket from Thai Airways Flight TG 585.
Without question this would throw up all sorts of immigration red flags because all Bangkok Airways would see in their system was a single flight ticket and no return date. It's highly unlikely there's any discrimination here - this is absolutely standard procedure for airlines across the world. The OP's contact is barking up the wrong tree here.