You may have to show PROOF of your DTV Visa to airline staff on check-in for your flight ✈️ to Thailand...even to the connecting airline staff. Or to the land border crossing entry Thai Immigration Officer.
Upon arrival into Thailand, show the DTV visa to the Thai Immigration Officer and look at the STAMP placed in your passport to be sure you are STAMPED in for 180 days.
That's WHY it's recommended that you PRINT OUT the DTV visa for presentation to the airline staff.
And requesting a supervisor review of the denial of the boarding pass.
That can happen at your initial flight ✈️ check-in as well as the connecting flights ✈️ to Thailand.
Every time I have flown to Thailand from the USA, I have been ASKED to see my Thai Visa. I sticky note the passport page for easy access to the stamped one year extension of stay as RETIRED.
In pre-retired years, I always had a one way ticket to a SE Asia country served by a Skyteam partner airline (Hanoi, Ho Chi Minh City, Seoul or Taipei), as I would book it using 20,000 frequent flyer miles...after arrival into Thailand, I would cancel the booking and the miles used were returned to my airline account without charge.