There is a class, System.Windows.Forms.ToolTip, that specifically says you
can specify line breaks with "\r\n". But, we have no idea whether that's
the class you're using. Even your own posts show two completely different
ways of setting the text, neither of which include enough information for
anyone to know what classes you're actually using.
Support for newlines in text is going to be very dependent on the specific
class you're using. Unless you provide full context (e.g. a
concise-but-complete code sample that reliably demonstrates the problem),
your question cannot be answered.