I found that on a FAT32 disk the tree command does not sort by name by default.
Instead, it seems that it sorts by time-stamp.
I have not been able to reproduce this issue on NTFS nor on network drive.
Thank You for the help.
Right now I have not access to that system, but I put here a part of the on-line help, that says the same thing.
It says, regarding the /O option, that by default the tree command sorts by name.
So, maybe it is an issue of the documentation, not of the software.