When a parameter has annotations and also has a line-wrapped description, like this rather contrived example:

 /**
  * annotation_nullable:
  * @uri: a uri
- * @label: (allow-none): an optional string
+ * @label: (allow-none): an optional string, which is used in ways too
+ *  complicated to describe in a single line, making it necessary to wrap it
  *
  * Document optional parameters.
  *
  * Returns: (transfer full) (allow-none): Returns stuff which you have to free after use
  */

the HTML output loses everything after the first newline. The same parameter with no annotations works fine.

The single-character fix is to change gtkdoc-mkdb.in like this (I'll attach in git format once I have a bug number):

@@ -2623,11 +2623,11 @@ sub ExpandAnnotation {
                     "unknown annotation \"$annotation\" in documentation for $symbol.");
                 $param_annotations=$annotation;
             }
         }
         chomp($param_desc);
-        $param_desc =~ m/^(.*)\.*\s*$/;
+        $param_desc =~ m/^(.*)\.*\s*$/s;
         $param_desc = "$1. ";
     }    
     return ($param_desc, $param_annotations);
 }

I haven't yet verified whether long, annotated Returns lines work.