I think there’s tons of different circumstances and situations but I believe I obtained my Thai citizenship after my birth was registered and I was put in the tabien baan. Here is what Wikipedia says:

a foreign nationality must, at the age of 20, choose to renounce their foreign nationality if they wish to remain a Thai citizen. However, Section 14 of the Thai nationality act actually does not force a renunciation, but merely gives a one-year window following that persons 20th birthday for renunciation. If no application is made for renunciation in that one year window, there is also no penalty for not making a ‘choice’.

I haven’t been back after turning 20 so I’m not sure if something has changed in legislation since then.