How to run my Play Games with its correct game progress after it runs another Play Games account of the same game app?
Looks like the problem is finally found out. I guess it is because when my new phone downloading .dat files from the cloud, I immediately signed in account B. It was saved at the same location in my new phone as account A. The data then sent back up to the cloud. Therefore when I opened my phone B, it downloads the data back from the cloud to my account B.
Lucky enough, I have backups of the .dat files somewhere else very long time ago to restore it.