Which version of 8 do you use? Latest number starts with 8.0.3. I cannot remember this bug, but first want to check if you installed the patches.
Are the styles that you show Custom styles? And you gave those styles the same name in the original Theme and the theme that you use to replace the original one? In that case, I don't think it is a 'bug' but by design none of the styles are deleted. Only the 'Default' styles, between brackets, will be replaced. I tend in a custom theme to assign custom styles as much as possible to Default styles, maybe that is the reason why I didn't see this really as an issue. I thought themes were meant to save time, and default styles, when applying a new theme, will immediately result in having changes in the project. For all other styles you do have to apply them again, right?