I have seen my error: rsnapshot was as comatose as I was at 0430 on January 01. No monthly backup took place, so nothing missing.
I did an out of cycle rsnapshot monthly, and monthly.2 was created nicely.
I have reordered my cron timings to prevent hourly from running into daily, but there had been no collisions anyway.
00 */4 * * * root /usr/bin/rsnapshot hourly
50 3 * * * root /usr/bin/rsnapshot daily
40 3 * * 1 root /usr/bin/rsnapshot weekly
30 3 1 * * root /usr/bin/rsnapshot monthly
One mystery remains to me however; monthly, weekly, daily and hourly rsnapshot operations all begin and end like,
mv /mnt/volume12/snapshots/hourly.5/ /mnt/volume12/snapshots/_delete.nnnn/
/bin/rm -rf /mnt/volume12/snapshots/_delete.nnnn
and according to /var/log/rsnapshot this has happens every time, except that every single day, 0800 rsnapshot hourly misses out the hourly.5 to _delete.nnnn stage.
I see nothing special about 0800. It seems a it may only cause a minor issue if I ever have to recover from a system crash at 0801.
But what the heck is it about? My original problem is solved (thanks syg00), but if anyone has a thought about this I would welcome it.