Hello,
All the issues you describe indicates a JavaScript error in the page which stop the execution of the JS.
In such case, best is to open the browser console and check for errors.
But in your case, I suppose it's something related to your template.
In the "Network", we can see that the content loaded do not include JavaScript.
The last line visible in that content is
<input type="hidden" name="data[variant][product_id]" value="546">
In the "productmarket/variant" view, after that line, some JavaScript is displayed so it can be processed by the page after the block is received.
If there is no JS in the page, the elements can't be initialized.
Seeing the source code of the page, we can see that "inline" JS is added in the "head" section of the page ; which mean that the content is modified by a plugin.
Regards,.