Hi, this is regular behavior sadly.
It has been discussed at length, for example here or here.
When you change the property of a Variant that has Variant children, and that property change affects directly the children, these are swapped to their default Variant. Figma does not identify that the property of the swapped children is the same as the original’s, in order to carry the right value over to them. It behaves like the swapped children is a completely different component.
As a general fix for theme Variants, I would advise to manage theming differently (using a plugin or the “Swap libraries” feature). But I know it’s a huge change, and does not solve the original issue.