GRUB not showing content of "/etc/grub.d/30_os-prober", but grub.cfg has it

[Art Grandi]

I have, against my will, been unable to answer until now.

I want to thank  for your attention and inform that I consider this matter
solved.

I missed an existing stanza for Void in the Grub menu, due to a layout in
Lubuntu which differs from any one I  knew.

As for Lubuntu itself I have come to the conclusion that Grub places one
only menu entry  for partitions with identical kernel and initramfs. If
this is absurd I would appreciate hearing about it. This understanding was
based first  on the observation that os-poberinfoms that it has found both
(identical) kernels and second on the fact that by editing the only menu
stanza for Lubuntu and  replacing the "root+=" parameter with the value for
the other partition with the other version of the same kernel I was able to
swiftly boot in that other partition