I guess a total 'ban' until 6/12 of line flying may be an effect of trying to make rostering more straightforward, even though the AAIB report only refers to landing.
I'm a bit out of the loop at the moment and have no "special " insight
into this but knowing how fluid the operation at BA can be I would have thought a total ban on
operating the A321 with < 6/12 would make rostering a nightmare. Day to day ops, most especially anything requiring a last minute equipment change would be a minefield, given the number of down route crew changes you have around the network.......
"Oooooh look blogs, they've sent us a 321......."
A landing restriction makes more practical and operational sense to my tiny mind but those on the fleet in question I'm sure have chapter and verse.