Can you explain how the 1/8" bleed works in InDesign for the gutter margin?

Not sure if your PDF is exporting correctly? Seeing a "spill over" occuring across the pages?

When creating a file in InDesign set up as spreads there is not a gutter to expand images into. So when exporting the file with 1/8" bleeds the gutters of the pages will take 1/8" from the opposing pages to create the 1/8" bleed.

This is not a concern as we'll crop these gutters and ensure the cut/gutter ends up on the trim edge during our printing/binding process.

Also see: