When you release the mouse button on the planner while holding Ctrl+Shift (for super-force-build), the render path remains in the world. Using the planner again continues using the old path, as if you never released the mouse in the first place.

Not sure if it's supposed to be a feature, but I didn't see anything in the source acknowledging it.

Also curious, how do you plan on supporting the other types of transport belt?

Yeah i knew about this because the API was missing an event, i even submitted a bug report for it: https://forums.factorio.com/viewtopic.php?p=688616#p688616

but ill try a different approach to fix it.

I think other belts will just be a separate tool.

Testing in 2.0.1, I've noticed pressing Pipette (default key: Q) while drawing the path does the same thing: it leaves the render behind.

EDIT: And I get the "Cannot change belt tier while dragging" message if I try to switch it in this state, which I suppose is to be expected.

Will be fixed in the next release. All though due to some limitation, to get the proper behavior youll need to disable smart belt building and toggle a setting in the mod settings. Otherwise it will fully clear your cursor even if you Q while drawing.