I’ve been able to fix it by removing the interface from blueprints that use it, then deleting all bugged functions within the interface, recreating them and then adding the interface again.
Still encountering this (version 4.10.2) - just for renaming an output variable inside a blueprint interface function.
Given that readding the interface (breaking everything which uses it) seems to be the only solution, my current recommendation is that for whatever functionality you are using the blueprint interface, contain it all within a separate function, so that the process of readding is simply connecting up the pins between the function and the interface in each blueprint using it (which is still a massive amount of work).
Outside of that, either lock down every variable and every variable’s name in advance of using the function, or avoid using blueprint interfaces entirely.
Solution is to right click on the function on the list on the left and then choose ‘convert to event’ that will get rid of the junk old function that doesn’t return anything anymore