Hah, today I found out...
... what 'Workshop' means in the Filter. Apologies to those already in the know, but it isn't exactly obvious (least not to me it wasn't). Someone will no doubt point out where this is clearly stated but whatever I had no clue.
It means 'minimal effort' maps. Apparently each map you create (or copy) comes with an operations count - allegedly around the 200 mark. So if you create a new map from scratch and only do say 150 things to it, when you publish it doesn't go into the normal group of maps but into a 'Workshop' collection. Same is true of the myriad clones you see out there. If you don't amend it sufficiently then it too will never see 'Newest'.
I fell foul of this when Ubisoft produced a great map called 'The Forbidden Zone!' which I loved, apart from the fact that all the weapons were loud, so I simply recreated their map/loadout but with added suppressors. Called it 'Forbidden Zone Quiet'. Having only messed with that needless to say it went straight into 'Workshop' to be more or less universally ignored. Heh heh heh. I wondered why it never showed up under 'Newest', now I know.
So I guess the moral of this is if you intend to copy someone else's work make sure you put enough changes in for it to count sufficiently as a new map in its own right. Better yet, do your own not simply filling an empty textureless map with two dozen Yetis and call it done.