Theory, but please confirm mono --version is still per below (from July 26, 2019). Docker mono is newer.
Added support for nanosecond resolution in file information on platforms where the information is available. This means the return value of APIs like
FileInfo.GetLastWriteTime ()is now more precise.
Synology seems to use btrfs or ext4. Both have nanosecond resolution. Use stat command to see it.