It's important to debunk the idea "Double taxation agreement = no problem".
Because it's false.
I give you a real exemple.
A Thai tax resident who receives dividends from a company located in France... will pay automatically income tax of 12,8 % (to the french tax department).
Yes, it's a flat rate (he could opt for variable rate, but it's another debate).
(reminder : for the flat rate, a french tax resident... will pay 30 % on the same dividends, because he has 17,2 % of social security taxes added on top).
So according to you, there is indeed a DTA between France and Thailand, and taxes have been ALREADY PAID (to the frenchs). So everything is okay.
It's not.
The franco-thai DTA says (article 23, 2 B) :
Thailand grants a resident of Thailand receiving such income [dividends] from French sources a chargeable credit against Thai tax. This credit is based on the amount of tax paid on France, but it cannot exceed the fraction of the Thai tax corresponding to the ratio existing between the net income from French sources and the overall net income liable to Thai tax.
In other words... sure the thai tax department will give you a "tax credit" (of the amount paid to the french)... but can tell you TO PAY MORE (aka 35 % - 12,8 %, in case of dividends higher than 4 millions THB).
References : the DTA
https://www.impots.gouv.fr/sites/default/files/media/10_conventions/thailande/thailande_convention-avec-la-thailande_fd_1970.pdf
The thai tax rates :
https://www.rd.go.th/english/6045.html
Let's sink that : a DTA is designed to avoid to tax 2 TIMES.. THE SAME AMOUNT... But that doesn't mean that one of the 2 states could not tax MORE ! ;-)
And rates can vary greatly between countries, and type of income, amount of income etc.
So again, a DTA is not the iron clad guarantee that everything is and will be okay.
Each individual must assess precisely his situation.
The rule-loophole of "year + 1" (for the money coming into Thailand) was a blessing (for the case I've described, the thai resident with french dividends).
But now the party is over (if they succeed to close this loophole or if they don't change their mind).