If you're building units or buildings that require both radioactive and metal, but get down to 0 radioactive + a negative amount, then the metal side will fill up.

this is due to the fact that the building or cruiser that you're building is radioactive locked, so it also won't take up metal since it's not using as much radioactive.

I never thought of that, it makes soo much sense now, thanks for clearing that up 

I don't know that I would call it a bug.  There are two different numbers - the demand you have scheduled, versus the actual resource net consumption.  If you have resources banked, those numbers are equal.  If you run out of something, you want to know how oversubscribed you are - not that your net is 0, so it's showing the right thing.  The only case where you really want to know the actual net is when you are oversubscribed on the opposite resource, but it could still be useful to know the scheduled metal demand as well.

So what I think would be nice is to see both scheduled demand and actual net, but only in the one case where it actually matters.