You are going to get a bunch of conflicting answers here - some based on "theory" of how things work, and some based on practical reality.
I have spent some time with a retired senior Thai diplomat and his wife. His last post before retirement was as Thailand's ambassador to China. He previously served as Thai ambassador to Kuwait, North Korea, Mexico/Carribbean, and several other countries that I now forget. He told me that over an entire career, he had never heard of a Thai citizen losing Thai citizenship. However - he had personally signed off on thousands of applications for new Thai passports, sometimes for Thais who had left Thailand 40-50 years earlier - and who had left the country without clearing Thai immigration (or without ever having even had a previous passport). There was a procedure they used to track down someone's national ID number - that is the only thing needed to reestablish citizenship identity.
He also indicated that many Thais travel on multiple passports, and that Thai embassies overseas are accustomed to issuing Thai entry visas to Thais who will visit Thailand using a non-Thai passport - but that such Thais must follow all the same rules as non-Thais, while they are traveling as such.
My acquaintence is presently a retired diplomat who recently served as an Associate Judge of the Intellectual Property Court of Thailand - until being hospilatlized for bypass surgery.
Let's see what the rest of the rabble comes up with! (By the way, my son, born in Bangkok, is a Thai citizen as far as Thailand knows, but he is also a US citizen - with a US passport).
"Let the good times roll!"