Here's what I'm seeing.
This is correct behavior when I use medium taskbar (Start11) and automatic distance from edge (ObjectDock). My dock sits on top of taskbar. Dock is slightly shorter than full screen width so you can tell the difference between dock and taskbar since they're the same color. I have two screens. Dock is on first/primary screen. I'm only showing screen shots for primary screen.

When I open a maximized window, it sits above the dock. This is correct. Taskbar is still medium.

When I change the taskbar to small (instead of medium), at first it looks correct. This is still automatic distance from edge.

If I open a window (not maximized), the dock moves upward. This is still small taskbar and automatic distance from edge.

If I open a maximized window, the dock stays (or moves) upward and the maximized window is behind the dock. The window sits on top of the taskbar instead of the dock. This is still small taskbar and automatic distance from edge.

If I change the dock to a manual distance from edge, the dock stays in the proper position but the maximized is behind the dock.

All problems are related to the small taskbar. When medium taskbar, the dock and maximized windows are positioned properly.