Pretty sure there's a wall behind her which only blows up when she dies, preventing that from happening
Though I don't think I've ever seen anyone get back there without beating Mother Brain anyway. Maybe part of the terrain conveniently falls apart during the self-destruct sequence which would prevent going down that way earlier
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(10:17:09 PM) Ed Bellis: i think pac-man just keeps repeating, for instance
(10:17:26 PM) ertyu: ya well tell that to billy michtlle