This was reported by a Discord user:

Once the color bar splitter is fully moved to the right, we cannot move it back to the left:

If I try to drag the right border it drags the window border, not the splitter, then it resizes the window. There is no way to drag the splitter again.

But there is a workaround: open the aseprite.ini file and edit the following entry as:

color_bar_splitter = 80

This was only tested for the macOS version, not sure if this happens for the Windows and Linux versions.