Semantic UI - Why no message inside a Form?

Well, I'm not 100% sure, but I'm guessing that the error / success / warning classes are used for form validation messages, thus the reason they are hidden by default. I solved the issue by using the specific color class rather than the warning one:

<form>
    ...stuff...
    <div class="ui yellow message">
        <div class="header">
            Title of the message
        </div>
        Text of the message
    </div>
</form>

This works perfectly and is visually indistinguishable from the warning one.

I also had the same issue. Not sure if you already have the answer, but for others visiting this post:

wrap you <form> with <div> with class "ui form error".

E.g.,

<div class="ui form error">
<form class="some class" method="POST" action="/some_acton">
..fields..
</form>
</div>

or

<form class="ui form error your_class" method="POST" action="/some_action">
..fields..
</form>

Adding visible class to the message div works for me. Try this,

<div class="ui warning visible message">