The reason XOOPS lowers the message you paste is that TextSanitizer has convert all line breaks (PC style \r\n, Unix style \n and Mac style \r) to <br> automatically
A suggestion: add an additional XOOPS code (like i.e. [ HTMLONLY ]) to core system. Then, all text inside the HTMLONLY tags will be text-sanitized in a different way, avoiding to switch newlines to BR automatically.
In this way, you can still use easily your short HTML tags inside a text, but if you want to insert large HTML codes you can put them between this new tag, so they will be showed exactly as they are without any exceeding BR .