XHTML 1.0 Renderer produces invalid HTML for FigureCaption blocks

To reproduce:

• Create a page with the following content:

  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
  
  {{figure}}
  [[image:whatever.png]]
  
  {{figureCaption}}Themenblöcke des Validierungsdokuments{{/figureCaption}}
  {{/figure}}
  
  YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY
  

• Edit in WYSIWYG and click source or save
• You'll notice that "YYY*" lines has disappeared

We need to find out if the issue is caused by some wrong Blocks generated by the Figure Macro or some generic issue with the WYSIWYG/Rendering.