I agree it is well worth a check on exit/entry stamp.
I have not had Thai visa exemption missed off before but have had reduced time like fouteen days when should've been thirty days visa exemption. I'm British, arriving at Suvarnapumi from an international flight (Manchester via Dubai) a full year since previous visit.
I only noticed the short visa waiver when in a taxi from the airport. Think the officer must have used the wrong stamp.
The reduced stay time prompted me to make a border crossing into Cambodia and on return to Thailand had a new visa exemption stamp which covered me for the remainder of my holiday.
Don't know if could have requested a thirty days stamp. Perhaps the immigration officer's decision is final? I suppose I might have tried to suggest, politely, I would rather spend my holiday time and money in his country rather than taking my tourist dollars elsewhere.