when I put stacked fuel ( ex 3000 ) on miner cap or other, it charge 500 on it and the rest disappear, is a rust bug or plugin?

I have never noticed the excess fuel disappearing , if you try to add more that 100 fuel the rest is put back in your inventory, If  you use "NoFuelRequirements" plugin you only have to add a minimal amount to the hat and thats it.