Emma here from Shopify. Welcome to the Community!
Are you using any apps for displaying your product variants? It looks like it could be an issue with an app. Otherwise, it's likely in the theme but it's hard to say without looking further into the account.
If you could let us know if you're using any apps for the variants as well as which theme you are using, that will help in determining where the problem lies.
Looking forward to hearing back from you!
Emma | Shopify Support