It looks like you have something representing a small airgap with different permeability on each side (I'm not positive about the permeability change).
If that is the situation, it is usually critical to have very fine mesh in the small airgap area. For example max triangle side less than 20% of the gap. You can do this by creating a rectangle to represent the gap area (you already have two sides, just close off the other two). Put a block label in there and ajdust block label properties to specify mesh parameters. You can often see significant change as you decrease the mesh size. When the behavior stabilizes with decreased mesh size, then you have reached a good mesh size.
I apologize if this is not relevant to your situation, or is already understood.
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(2B)+(2B)' ?