It would actually need to know that the real-time mirrored object was attached at vertices A,B,C,D... etc in order to not subdivide along the mirror edge.
That's more information than the program cane safely assume.
If, however, you had an open edge which would be preserved (or only subdivded in the plane of each open edge) then you could get good continuity for the mirror as well as for objects which you wanted to attach later, or keep separate along a clean line, etc.
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