[Invalid] Source file timestamp incremented by 30 days after syncing

[player]

FFS version: 10.24 64bit 05/17/2020
OS: Ubuntu 20.04
Source disk filesystem: exFAT
Destination disk filesystem: ext4

Today I used FFS to backup my data. After syncing is done, I clicked comparison again. A bunch of files showed up, even ones that were not listed before syncing. I inspected and found most files from the source are newer than destionation files by 30 days. I clicked syncronize again anyways and the same thing happened again. Now I have a bunch of files with timestamps in the future.
I thought it's a bug in ubuntu but when I copy a file using file manager between the same pair of disks, the timestamp is unchanged and preserved.

[player]

OK nevermind. It's a ubuntu bug. The timestamp changed after unmount and remount even when copied using file manager.