This always annoyed me, but i finally was able to give exact description:
In VFB editor window>Tone mapping>exposure... let's say I have a value of -1.5 for my EV, and now I want to adjust this with a new value, so I double-click the input field to select everything.... except the "-" sign is NOT selected. So, normally I would quickly input a new value - let's say -3, to make the adjustment.... but the value is not accepted in the field - it stays the same as before, except now the whole value, including the "-" sign is selected, so if i enter "-3" again, it will work this second time.
If the first time, I just enter "4" it will register as "-4", because the "-" sign is still there.
I know the work-around is to not double-click, but instead use the backspace to erase the existing value, and then enter the correct value... but who has time for that? LOL
I know this is probably the most ridiculous bug report ever, but it has caused my frustration to no end ;)