Unity draws things a few different ways, regardless of ortho vs perspective.
The default 3D Renderers draw stuff according to Z depth - distance from camera.
Sprites draw according to Sorting Layer and Sorting Depth (properties of the Sprite Renderer)
UI Canvas Renderers draw in linear sequence, as if you were pasting papers atop each other
When you mix and match you can get these issues. There are workarounds for all combinations. Google up mixing sprites with 3D objects, and mixing 3D objects with UIs.