A font can contain tables which instruct it to replace certain combinations of characters with a glyph that represents them written together as a ligature. You won't come across it that often, and might not have noticed when you have. The image below is an example of this with the Tekton Pro font with the letters "ff" together and separated by a space. Note the ff is a glyph with one continuous line. If it were two normal f glyphs, that line would be staggered.
For some reason the Purisa font has such a substitution for "TM", replacing it with an entirely different glyph. This may be a designer's signature or some such whim. As it is primarily designed for writing Thai, perhaps TM wasn't thought a likely combination for someone to want to type.
This is a version of the font that I have edited in fontforge, and removed the instruction to substitute TM. You could use this to replace /usr/share/fonts/truetype/thai/purissa.ttf as a quick fix (after replacing the file you need to run
fc-cache -r and might need to log out if you are using it as a system font). I've filed as a bug.