OK I can can understand how it wouldn’t be considered a bug if it hasn’t been implemented yet, but because it is a behaviour inconsistent with the rest of the UI, at very least it’s been overlooked and can cause confusion.
The interface should be handled exactly the same way one sets different styles using other controls: that is, when a style (like width) is set for a larger breakpoint (like “Desktop”), that style is displayed greyed out in the input field after switching to a smaller breakpoint (like “Mobile Portrait”), indicating that the style will be inherited but can be overridden in the smaller breakpoint.
Here’s how I imagine this working with Custom CSS. First enter some custom CSS at the Desktop breakpoint:
Then switch to a smaller breakpoint. Notice how the Custom CSS field contents are grey out, indicating it will be inherited at this smaller breakpoint, but you can enter different CSS rules for the smaller breakpoint which will apply to this breakpoint only. Rules may or may not override the larger breakpoint’s rules (depending on what the rules are).
I’m not sure how you can say the percentage of people who set different styles at different breakpoints is tiny…I mean, I have done it on virtually every site I’ve created.