I don't know the exact reasons in your case.
I have observed occasionally that dock-areas go "outside" the main window. Then the splitter can't be reached anymore.
Some reports said that happened when dpi was changed. But I don't know.
I recall doing some attempts to force the splitters inside, but IIRC that didn't go as planed. Its a while back, not sure if any of it went into 4.99 or not. Would have to check all the commit history. But even if, that wouldn't fix it, just attempt to reduce the damage.