By default, in SharePoint display form the multiline text field is written as a string without html-tags. To display text as the multiline one with HTML-tags, you should modify the default parameters of the XSL expression.

This code is the default:

<xsl:value-of select="@comment"/></span>

You should add  'disable-output-escaping="yes"':

<span><xsl:value-of select="@comment" disable-output-escaping="yes"/></span>