Hi,
It seems there is some bug with the variants switching on the page.
However, we don't have such issue on our end with a similar setup with the current version.
So I'm thinking it's probably related to a view override on your end.
Check the folder templates/YOUR_TEMPLATE/html/com_hikashop/product via FTP of a file manager provided by your hosting.
You should find view override files in there.
Try renaming that whole folder to something else to deactivate all the overrides in there and see if that helps (clear the caching you have, if any).
If it does, then the issue comes from one of the files in that folder. You can revert the name of the folder and then change the name of the files in there one by one to check which file is causing the problem.
Then, either leave it renamed or check with the person who made the view override (your template provider maybe ?) if it changes your layout in a way you don't want.
If renaming the folder doesn't change anything, then it might be linked to the magic toolbox extension you're using. I know they've added one or two patches regarding the variants switching mechanism in 2020, and you might not have these patches, or it could be a new issue. So you could first check that you have their latest version, and then try disabling it to see if it's linked to that extension.