I had exactly the same experience with Bangkok Bank a few years ago.
I opened a Foreign Devices account with the head Bangkok Bank in Bangkok back in 2004 in order to get my Visa based on Retirement.
The required sum was 400,000 Baht.
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A few years later, when the required bank deposit, changed to 800,000 Baht, I went to the same Bangkok Bank to top-up my balance to the new requirements.
The same lady who opened my account told me that the bank rules had changed and I needed to get a "Letter of Reference" from the embassy and a proof that the money was legally earned.
I tried to convice them that I was not asking for a new bank account but only top-up my current account, but only to meet a "NJET".
I had an account since day 1 in Thailand, but they had refused to open a Foreign Devices account in 2004 unless the minimum deposit was meet (4 million Thai Baht).
I went back to the UOB bank to ask to open a Foreign Devices account and that was suddenly no problem.
Thailand or "Change with the wind".