When I mark a buffer for saving (S marker in the 3rd column), the unmark does not clean the flag.
This happens because the unmark does simply so:
(tabulated-list-set-col 0 " " t)
and the "S" marker is on the 3rd column, and it's not cleared.
Now I discovered that in fact "Unmark" command should be called "unmark the deletion".
Info page, 19.5 Operating on Several Buffers, quote:
Remove all flags from the current line, and move down
This is not true. Either press "g" to revert the buffer, or re-define "Buffer-menu-unmark" to really unmark the buffers.