Hi,
Maybe it's a bug of the 2.4.0 ?
It's already two years old.
First, check that the order admin notification is enabled and that you have an email address entered in the "order admin notification email address" setting of the HikaShop configuration.
Otherwise, the email might not be sent.
But since you get the email if you turn off the HTML of the email, it would indeed indicate a problem with the HTML version, like something linked to the overrride.
With a recent version of HikaShop, you could go in the menu Customers>Emails history and check whether the email is listed there or not. If it would be there, it would indicate a problem with the email server, if not it would indicate a problem with the generation of the HTML of the email.
Check PHP error log of your web server, you might find a fatal error message which could explain the problem with the HTML version.