I'm using Code Composer Studio Version 5.4.0.00091 and of course the problem is still here as the bug ID is still considered a known issue. Honestly this is quite the annoyance since it affects my CLA assembly code readability. There are a number of other characters that are treated as line comment characters:
! @ # ^ @ * + = - [ ] < > | , ? | ` ~ ;
Digging deeper on my spare time, it seems the getLineCommentCharacters() method from TIASMLanguage.class is working properly; in fact, I can change the line comment character from a semi-colon to the letter A. After reassigning the line comment character to 'A', semi-colon is still treated as a line comment character even though 'A' is as well (hence I added semi colon to the list above). Something else is going on in the lexer to make the parser think those characters above are line comment characters.