Reproduces on Master (481375fefe)
reproduces on 9.x.x/release (2ddf9def91)
doesn't reproduce in 8.x.x/release (4b3e9d383d)

Repro steps:
1. Create a "Float" (or "Vector 1") node, leave it as 0
2. Drag out the output of the float node, when the searcher opens choose "Divide" with slot B
3. Plug the float node's output into the Divide node's A input
4. Delete the float node's connection to the Divide node's B input
Expected result: the preview of the Divide node should be black, because it's 0 being divided by 2
Actual result: the preview of the Divide node is NAN indicating that B is still being treated as 0 even though it displays 2
Note: changing the slot value for B to some number and then back to 2 will make it behave properly