Hi, no that's not an issue that anyone else has reported nor is it reproducible here.
Are you using our default JAR dictionary, or are you explicitly setting the path to a .dict file?
If it's a .dict file, then you can add words using the Dict Manager tool (it's available in the product ZIP you downloaded - also see the Help section on dictionaries). But. I doubt that the problem is that the word isn't in the dictionary.
Perhaps you can try reproducing it in a basic page (eg using one of our demo pages, the demo app is in the product ZIP).
Posting as much useful code as possible, and also the actual text you're checking will help.
The only time "Thanks" would be expected to be underlined that I can think of, is when it's a duplicate, "Thanks Thanks", or if "thanks" starts a sentence.
-your feedback is helpful to other users, thank you!