Hi,
On my pc, it works fine on FireFox and Chrome, but it doesn't work on Edge.
So I compared the HTML seen with the inspector tool of each browsers.
Here it is on Chrome:
i.imgur.com/AJZFBnB.png
And the same on Edge:
i.imgur.com/bf0KqEL.png
So if you compare the two, you can see that inside the main div of the product page (with the itemscope attribute), you have a "form" element on Chrome, which contains the div with the class "row" (and inside all the HTML visible on the product details area of the page), and then some hidden inputs and that's it. And after the form element, you have divs for the variants (with the display:none; inline style on them).
However, on edge, that form element is missing and the HTML of the elements is directly below the main div of the product page and the divs for the variants are also together at the same level.
What this means is that browsers have a hard time parsing the HTML of the area because there is some invalid HTML tags.
And by default, the HTML of HikaShop's product page is valid. Normally, this indicates a problem with either the HTML of the product description or with the customization made to the product page.
Besides that I don't know what it could be. I would recommend to double check your overrides and make sure you clear your website and your browser's cache when you do.