The philosophy of mass of course is that mass is absolutely real (whether the universe is simulated or not).Your whole argument is a straw man.
Mass is an absolute as in actual, as in a real, tangible, quality of the universe. Whether our previously chosen measure of mass - an actual, real, lump of mass, might in fact lose some mass over time or "decay" is a trivial irrelevancy and fixable by using a different definition such as the one currently under proposal.
All was explained above.
The fact still remains that in science there is no such thing as an absolute quantity which contents could be known. You can choose a fixed measure of 1kg and see how it feels in your hand relative to something else, perhaps two times 1kg.
There is no way for you to have a perfect absolute constant of 1kg that would always be the exact same. You have no measurable absolute constants in science. You can have hypothetical absolute constants though, but then you will never know if they're truly constants. They just feel constant to you.