Pretty early in a game, I noticed my Isotopes were stacking up much higher than expected.
I noticed I had this level 3 mine running. It had 10 iso and 20 minerals in the storage. The cars that ran to the mine left with 6 isotopes each but the inventory in the mine never decreased. They just kept picking up more and more isotopes. In the picture you can even see a filled car just leaving.
Reloading seems to have fixed it.