Why the horizontal spacing is removed in a subsection when using special characters?

I’m trying to add a special symbol to a subsection of the document, but it appears from what read online "PDF standard doesn’t support special symbols in bookmarks". I found the answer and someone suggested to use texorpdfstring{}. It worked, but I also wanted the symbol to be displayed in the bold letter so used the bm{} package. It works but when I compile it removes space from the next characters as shown in the image below. Any help would be appreciated.

Code

section{Case 1 texorpdfstring{bm{$alpha}$} is kept constant}