The short answer is that a bold title is not what you think it is, nor can you just make it bold. In fact, the only reason your thread title would not be bold is because you've already clicked on the thread. Therefore, the forum has marked the thread as 'read' on your account, thus changing the thread title back to a normal typeface.
Newbie guide article: https://www.blackhatworld.com/seo/how-do-i-make-my-thread-title-bold.1291023/
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