copper plating issue, the copper isn't bonded to the base metal too good and that is a bubble which is very common on copper-zinc pennies find those a lot in change no added value
This couldn't have come off a case-hardened steel die. As PMD is ruled out, I agree it's a planchet plating issue.
Just when you think the anomalies have peaked on the Zincoln's, here comes another one, wow! Thanks for posting it.