It looks like the ink is not dried completely and that causes the problem. From our experience the ink will need approx 48h for a good drying, please keep also in mind that the ink should be suitable for the blister process.
If these both parameters are already checked, the customer has to play a little bit around with his sealing parameters. Maybe a little bit more heat or a little bit more time will complete the sealing process.
From the picture it looks like the coating is working, except this area on top.