Naked edge when solid filleting

Why it becomes naked edge when filleting solid piece with solid fillet command?
Both sides are the same because they are mirrored. Why only other side makes the naked edge?
Yes I can cut off the bad side and mirror the good one, but it should not act like that. Frustrating…

That shouldn’t happen normally in simple cases - can you post the objects in your images before the fillet? (plus the radius you used)

I normally use “Variable radius surface fillet” (! _VariableFilletSrf) for single fillets. It’s much more stable and usually gives cleaner output surfaces, though it may require some extra manual trimming.