Because of the way beacon fuel was designed.
If you put fuel in ANY length of time before it expires, you are wasting fuel. (This is horrible for people that try to max efficiency like me)
They should have had beacon fuel ADD to the time, but put a limit on when you can refuel again like you can only fuel a beacon until the remaining fuel is less than half of the amount you are trying to fuel. That would work with all fuel types.
For example.
If you were trying to refuel a beacon with one that lasts 2 weeks, you would have to wait until it has 6 days, 23 hours and 59 seconds for it to consume your fuel to have now 2 weeks 6 days 23h.
As well if you put a 16 week fuel you would have to wait till there is 8 weeks left to top it up to 24 weeks.
Of course the amount of time per fuel type can be tweaked for balance or whatever.
The problem with the current system is that if you don’t want any losses you have a window of exactly ZERO seconds to refuel before your plots go wild.
What I am proposing is that window gets expanded to half the amount of time of fuel you are using. (Or about to use, or whatever.)