As concerns print through or transfer of one part of the ballot to another, either at the original manufacturing process or while being crushed in folded form in postal sorting machines, or stacked after receipt at the county... it is important to know that many ballot manufacturing processes are similar to laser printing... ie toner is used, and toner is subject to transfer from one place to another when heat is involved if not in other circumstances. Hart manufactured ballots seem to sometimes have toner transfer onto the removable stub on the back side, in this election.
It is not the case that ballots are necessarily printed with ink. However, it is useful to know if this is the case. I understand Boulder County Colorado ballots may have been manufactured or printed by some company other than Hart.