If I press the max button on the Hypercube Hept while my offerings are high, then press on the buy button after AutoRune spends the offerings (making them below e80 * # of hepts to craft), I get to max this hept and make the offering amount negative. Reincarnating doesn't seem to do anything (perhaps because the offerings I get on Reincarnation doesn't do a dent on it; I haven't tested ant sac) but it fixes on refresh, going back to 0.
In the first instance of the bug happening to me, I somehow managed to craft a negative amount of hepts too. I have no idea how to replicate it.