Why is my battery pack self-discharging?

I have a 4Patriots D320 powerbank that was sitting unused and not charged for a couple of years. It would not charge, so I removed the battery pack that consists of 4 series connected groups of 6x 26650 cells in parallel with a charge/discharge PCB. One of the groups was totally dead. The other groups were all about 3.2V. I removed the dead group. The rest are still connected to the charge/discharge PCB.

I charged each group individually with a simple smart Li charger at its max of 1A. They all charged up to ~4.15V. After a day, the voltage on all the groups was down to ~3.5V. I recharged and after 8 hrs they are ~3.8V.

Is this what I should expect, or is something else happening?