Resolved (I am dim)

Never mind, it's because if The second A would need to be mareked as green rather than the first A marked as Yellow.

I seem to have coppied the guess coding wrong it should be: Yy-y-(-)yyy--y-yg, backets around added character.

Y = Yellow
G = Green

• = black

Which explains why it appears I've missed the M. Now ZINJANTHROPUSES could be a viable word.

The guess:
SACRAMENTARIANS

Yy-y-yyy--y-yg

expects {'VOLUNTARINESSES', 'PENINSULARITIES', 'WITHDRAWNNESSES', 'PENETRATIVENESS'}

but I believe ZINJANTHROPUSES should also be included. I can't see why that would be exlcuded.