I was able to reproduce it but I don’t think the issue has been described correctly. If the cursor is on the line containing the characters of which you are changing the background, undo/redo works as expected.
If the cursor is on another line, when you do undo/redo, the background still changes but is not visually updated until you tap the line with the characters. So you see it change as long as the cursor is on the same line as the characters you are changing the background of.
I just noticed that it works fine only if the cursor is in the same line/block as the background colour; because the bug also occurs if you close the keyboard/editing menu.
I will update the description.
Edit: Looks like I can’t edit the original post. But moving the cursor out of the current text block would be the additional step for reproduction.