Hi,
When a product doesn't have a default unit, the Unit Selector still shows a unit selected, making the user think they are seeing the price of that unit when they're actually seeing the product price without the unit selected. If you then select the same unit that is already showing in the Unit Selector, the price changes to the actual unit price.
I think it's quite misleading. There should be some sort of default option, like 'Select Unit' instead of showing the first Unit on the list.
I made a video to show the issue:
https://www.screencast.com/t/Kwee6QWA66S5
Regards