That's depend upon then Offset distance and fillet size,
Please not clearly most of the unfilled area is in fillet location only,reason for this is- Fillet size in A-side surface is smaller than the Offset size. we Don't have other choice you have separately fill those area's in B-side surfaces.
One option is there, don't give any fillets in A-surface and then offset, then you can get clearly.
Note: we are not suppose to change anything in A-side surface,because A-side surface's from OEM.
I hope you can understand little bit from my answer.