So if this is an intended feature, two questions.
One, why is it specific to the Select Tile window and not also the Tile Editor Window? As mentioned, the Tile Editor window will skip forward to CSet 0 with "+" if it was on CSets 12 - 15, and will also skip over all those CSets from 11 with "+" or 0 with "-", meaning the only way to view the Tile Editor in CSets 12 - 15 is to scroll to those CSets in the Select Tile window and then open the Tile Editor. Pressing "-" on CSets 12 - 15 will properly scroll back to the previous one, rather than skipping back to 11.
Two, what exactly are CSets 12 - 15? I can tell a few of them are formed by grabbing colors from somewhere else. And I know these are the additional CSets in 8-bit tiles. Knowing what creates those colors, I could see some clever setups to get a "proper" CSet 12 and beyond that are formatted similarly to CSets 0 - 11.