Great wide field Andy! I'm hoping to pick this up with my wide field when the skies cooperate more so I'd be interested to see it all in context in time
As you're looking toward the centre of the Milky Way there are going to be large clouds of Ha that have not as of yet become dense enough to start another wave of star formation. Only small portions of molecular clouds ever become stars and the vast majority of any cloud gets blown away either by stellar winds of the newly forming stars or via supernova of ones that have formed not long earlier.
As Mike has said, the lack of obvious OIII or SII is due to the lack of recent star formation (supernova). Given its size and faintness and location (many objects within that region are only 6-8,000 lightyears away, I would imagine that it is a large molecular cloud that has been dormant for a long time but will eventually, due to gravity, become more dense and form another region akin to the M16/17 regions not too far away.