Create an item with a highlight color. Move the cursor to the middle of the item, hit enter to split it into two items.
Both items have the highlighted color from the original item.
The highlight color is only applied to the bottom item, and the top item is set to default (clear).
Chrome.
Split behavior is hard to design and has to be consistent.
Checked status and notes are not assigned to the new item either. Do you consider those bugs too? Anything inside the item can be split, like text and markdown, but highlighting is an item level thing.
I agree thereās rationale to both, and there will always be some people who found it not doing what they want. For that Iām sorry, but this behavior is intended and so not a bug 
As a workaround, you can duplicate the item (use shortcut) and then edit the text. Although Iām not sure if how much faster that is than splitting and then give it the color. It might even be slower.
Did not realize this was intended behavior. Unfortunately itās not easy to copy a full item on mobile, which is where this is mostly frustrating.
Oh, mobile. I see your pain point now.
Soon we should offer customizable toolbars on mobile, Iāll take note that duplicate is important (I think many others already echoed this before).