I have never heard of such a thing. Before focussing on the new diaphragm I would be considering that you previously directly soldered speaker cable to the diaphragm terminations. The heat transfer during this process via the diaphragm foils likely damaged/changed the delicate diaphragms.
I would tend to think the replacement is OK. If you haven't made any mods to the new one and think it is below par, then send it back for checking.
I think you are over-thinking it. The problem may well have been caused by your tinkering.
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