Titles, bodies and comments are formatted differently
Reddit has two kinds of text. Post bodies and comments support formatting: markdown in the Markdown editor, toolbar buttons in the rich text editor. Use that for bold, italic, headings, lists and quotes, since it is real formatting that every reader and screen reader handles properly.
Post titles support nothing: no bold, no italic, no links. A title is the only place on Reddit where Unicode styling is the sole option, and it is also the most visible text you write.
Styling a title without hurting it
Your title is how people find and decide on your post, through the feed and through search. Reddit search matches plain letters, so a styled keyword is not found.
If you style anything, style a short tag at the start, such as a bold "Update" or "Guide", and keep the rest of the title plain. Some communities remove posts with fancy titles, so read the rules first. Your 200-character profile about text is a lower-risk place for a styled line.