This is what I see in your output:
<span id="ProductPrice" class="product-single__price" itemprop="price" content="21.0"> <span class=money>$21.00 </span>
You see that the inner <span class=money> is missing the closing </span> tag. This leads to your entire form being inside the ProductPrice span! This span is updated with new price when you select variants and this is how your form disappears.
This happens because your money format is wrong. Check your Settings->General->Standards and formats, under the Change formatting link: your money format is
while it should be
|a minute ago|