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While I don't think there's a trope for the "flip flop", there are tropes for the individual concepts: Story Arc for the serialized bits, and Breather Episode or Filler for the episodic bits. It's hard for there to be a "breather episode" without something to take a breather from.
Half-Arc Season (although that's TV-specific).
Just a show with both Myth Arc and Monster of the Week episodes.
Incidentally, The X-Files is the Trope Namer of both.
A work goes back and forth between telling a connected narrative spanning multiple episodes and one-off episodic sections/chapters/episodes.
Edited by AudioSpeaks2