Hi,
I'll update my fellow iPad Mini 6 users here.
I spoke to one of the Apple on-line support and followed her instruction to re-update Os 15.5 again. After this is done, the problem persists. So I decided to bring it to Apple Store to get inspected.
Before I brought it in, I wanted to back up this iPad Mini 6 to my PC using iTunes. During this process, I found out, not only it does not charge, but iTunes also does not recognize the connection until I re-boot iPad Mini 6 again!
I brought this iPad Mini 6 to the Apple store, and one of their technician diagnosed my unit with their gadget and declared everything is working properly. In addition, while there, I was able to plug in and charge without re-booting! I told the technician that this is inconclusive, and at the end he agreed to send it in for me to get Apple's service people to take a deeper look.
Two days later, I got an email from Apple Support. They told me the unit was inspected, and they will replace it and send it back to me in a few days!
I should receive it in a few days, and will update everyone here. Although I am super curious to find out what was wrong with my unit!