Hi,
I can say that because no one else reported such issue with the guest checkout in a while and that I remember someone reporting that some time ago and us adding a patch for that.
Try this:
- Make a copy of the website, switch to the default template of Joomla on the copy.
- Test that you still have the problem
- Update your HikaShop
- Test that you still have the problem
Normally, you should not have the problem anymore on the copied website indicating that the problem has been fixed and will be if you update HikaShop.
You can try to replace the file administrator/components/com_hikashop/classes/user.php with the file from the latest version install package (in the folder back/classes/), but it's possible that it creates bugs because it is not meant to be used with your version. You can try that and revert if it creates other problems.