Hello,
I'm sorry but the configuration is not full.
Please provide a full example so we can reproduce your issue in our side.
Full example means product details, shipping method details...
So please provide all information so we can be sure and certain of what you have ; Thus we will be able to reproduce the issue.
Otherwise, please make test with a product without variants, see if you can also reproduce the issue and then provide us the full instructions to reproduce the problem (once again, with all details).
Without these details ; we cannot do it.
Regards,