We don't support the "code" bb tags. You're meant to use theÂ <> button in the toolbar to paste code.
Get-Content doesn't read your text as a single string. It parses the text, and essentially splits the file on CRLF. Each line in the file thus becomes a System.String object. When you set the RTB it's probably calling ToString() on those, which won't produce CRLFs.
Try doing something likeÂ gc filename.txt | Out-String to force the shell to produce a single string for you.