Anonymous participant they requested an additional document from my husband so his clock was reset back to 0. Like you, he was very anxious and impatient especially after everything changed to "pending approval" and he really wanted to leave Hanoi. He had to change his flight once due to needing more time in Hanoi than he originally anticipated.
My husband's application took a bit longer than usual due to a holiday. He submitted it on a Friday then the following Monday was a holiday. All in all, it took a total of 6 business days but seemed so long due to 2 weekends + 1 holiday.
Everything is fine until it isn't. Until you're stood in front of a particular grouchy IO that woke up on the wrong side of the bed or who's had an argument with their spouse the night before, etc.
Basically, it's really a luck of the draw.
What I CAN tell you from personal experience is that my husband is Singaporean. He'd never had an extensive history of coming/going to Thailand before Nov 2024. That's when we decided to move here from Singapore. We were both looking for jobs so had to rely on tourist exempt stamps for a while.
I eventually got a job but he hasn't so he'd been using the visa exempts for a total of 3x. On his 2nd entry, the IO was already asking him further questions but eventually let him go. On his 3rd entry, the IO explicitly told him he needs to get a proper visa or no entry next time.
My colleague also got denied entry and this was several years back. His passport was completely clean except 1 entry. His Non B had expired/canceled. He did a border bounce and was denied entry at the border.
I'm really surprised they issued you the DTV while having a still valid extension of stay (Non B).
I tell people time and time again that they need to properly cancel ANY work visas/permit that are currently valid prior to leaving. Or else there's going to be problems getting a new visa. Even going from one Non B to another (example: changing one school to another).
Good luck with canceling your Non B extension of stay.
It's too late in your case but just in case for anyone that's going to apply in the future, do what my husband did. Book a flight that is flexible/can be changed without any exorbitant fees. He booked this type of ticket with Scoot airlines and he changed his flight once since his DTV wasn't approved in time for the original flight. Luckily for him, it was finally approved a day before his new flight.