It is how it is assumed to work. It is definitely not a bug.

What you can do is to configure your attributes (e.g. Sales > Configuration > Products > Attributes):

• Open any and find the field 'Create Variants'

• The setting 'Always' assumes that in case any attribute value was added, Odoo would re-generate all variants

• The setting 'Only when product is added to a sale order' assumes that variants are generated only when they are needed for a sale

• The setting 'Never' blocks creating new variants

Thus, for example, if you decided to use 'Never' for all attributes, there would be a single variant per each template. Take into account that in such a case your inventories, manufacturing, etc. would also work with a single variant.