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.
If I were you, I would not trust the school to properly cancel everything unless you've been working for them for many years and they've demonstrated that they've done everything by the books the entire time.
The problem with trusting them to cancel everything without you seeing that they did is that they might not. Or they simply "forgot" or any other excuses.
Then what's going to happen is that even when all your paperwork for the DTV is in order, they'll have no choice but to reject because your Non B is still active.
The last time I left Thailand 3 years ago, I went with the HR lady to cancel both my Non B and work permit in one go and got my passport stamped showing proof of Non B cancellation.
You stated you had a work permit/Non B up until last month. This to me makes it seem like you had a job working IN Thailand. Perhaps they rejected it because it wasn't made clear that you've now switched to remote work with explicit permission that you're going to do this remote work from Thailand. Like the embassy thinks you're still working a regular job in Thailand. I'm conjecturing here because your post is a bit confusing.
Btw, I had a Non B for 7 yrs then moved out of Thailand. Even though I had properly canceled everything prior to leaving, when I returned as a tourist to Thailand many years later; the IO still asked me if I was working in Thailand. 🤪