Its because you are removing the background rather than changing it. I’ll need to tweak the class editor to handle that. I’ll see if I can get that in for 2.4.1.
In the meantime setting a 1x1px png on mobile will give you the desired effect.
You could use a class to load a bg image for desktop and tablet and then for mobile change the the image to a small transparent png (it can even be as small as 1x1px) and set the background colour to what you want.
Another work around is to create a seperate bloc just for mobile. Hide the mobile view on the desktop/tablet bloc and then hide desktop/tablet view on the mobile bloc.
You have save me a lot of changing css file. Because 2 people editing the site. And i did a css hack for mobile version. I just wrote new lines. But every time somebody exports the pages. It will be rewritten. So this solution is really fine. In my case i wanted to remove the background on mobile, and with transparent 1 px bg it works