Message mode registers message--syntax-propertize as syntax-propertize-function. This function is used by FontLock mode to highlight the buffer.
message--syntax-properties assigns comment-begin class to the ] on the second line. Because of this, it is ignored in the motion traversal.
But why is this? Why should ] at the start of a line be regarded as a comment? If > is regarded as a comment, it sounds reasonable because > characters are used to cite messages. But why ]? Are there any citing formats that use ] for citing?