was questioned when coming in visa exempt, when i was working in China, officers saw old B visas and just kept saying i was here to work, i explained i previously worked here but now in China, showed my contract but they just kept saying you here for work. On all these occasions i ended up talking to supervisors who quickly allowed me through once situation explained.
Under same entry conditions at fast track, never asked a single question.
I was in and out Thailand so many years due to working here and other Asian countries, used fast track many times as eligible due to flying business class. Never questioned by immigration once, yet when going thru general immigration line was questioned several times due to my passport having many old B Visas.
Fast track is exactly that, hand passport over and get stamped immediately.
If Immigration officer denies you entry ask to speak to their supervisor, I had this once in Phuket arriving on visa exempt and supervisor approved entry. Also once at BKK the officer questioned me after my 4th entry in 6 months that year and was taking forever to approve and looked every page of my passport about 10 times, I asked if problem or that I have done something wrong can I please see supervisor, he talked to officer next to him and stamped my passport with exemption stamp.