This is only my opinion but it is based on the premise of equitable taxation.
There are two components to withdrawals from Roth accounts: principal and income/profit. US won't tax either if withdrawals meet the required qualifications. Since Thailand doesn't recognize the special status of a US Roth account, they should only tax the income portion that has been earned since Jan 1, 2024. The principal and pre-2024 income portions should not be assessable income for Thai tax.