This is a request based on this thread on the older feedback.
Basically the tone mark is the most critical issue here. Tone mark characters are characters that attaches to the character before itself. However it doesn't seems like Text functionality of both Designer and Photo support attachment, thus results in improper text output.
To implement proper support for Thai script, please see the Microsoft's suggestions below (empasis on the Features section).
https://docs.microsoft.com/en-us/typography/script-development/thai#features
Also, you might want to implement a proper OpenType text shaping functionalities. Usually I'd suggest using library like Harfbuzz for text shaping, however since you're using .Net Framework on the products, I don't really know what is a good substitute for that (or even a Harbuzz-.Net bindings). You can also do P/Invoke as well I think.