I have a book layout established with a 3mm bleed on all four sides and several full-page photographs used throughout. I understand that many have issues with the treatment of bleeds when exporting a PDF from a spread layout, with suggestions to remove bleeds from one side, etc. I have also seen suggestions to leave the seemingly quirky results as they are, they will print fine and are intended.
What I want to know is, what is the purpose of InDesign placing a small strip of the opposite page onto the spine-side of the neighbouring page? It appears deliberate. Likewise, my full-page photographs have a strip of white now on their spine-side edge if the opposite page was text with a margin.
What is gained from this and will these strips be hidden in the spine binding, or are they sheared off as they're inside the 3mm bleed of the neighbour page? Otherwise I can see the concern that people have
Example of the bleed from the opposite spread page appearing on the spine-edge to the right (saved as cropped height, full width):
Edit: There appears to be some misunderstanding of elements of my description so I'm adding the following illustration with description:
In this spread example (in black) there is a 3mm bleed active on all 4 sides (in red). InDesign doesn't visualise the inner bleed so I've placed a pink and green line line where the bleed may be potentially located (if it's not using the inner edge as both an edge and a bleed edge).
The full-page image (blue oval) is positioned to meet the 3 red lines, fully covering the bleed area. For the inner side, the image is positioned to the page edge.
On the three outer edges, the output pdf shows that it will trim away anything that lays between the page edge (black) and bleed edge (red), as expected. On the inner edge, the output pdf has copied (not trimmed) a strip of the image 3mm inwards from the spine (up to the pink line) and duplicated it onto the bleed area of the neighbouring page, nested against the contents that exist on the page. Let's call that duplicated strip an orphan.
What is the purpose of duplicating the orphan to the opposite page? What function does it serve in printing, ignoring whether it was correct or not to include an inside bleed? Why did InDesign not just omit it in this particular circumstance or include it in a bleed area on the same page of the PDF? What dictates that the bleed was now 3mm inside the page edge, but not visualised as such? This is what I meant by an irregular rule. It doesn't follow the logic of the other 3 sides, though I am aware that InDesign has a different problem to solve with an inner bleed.
For what it's worth: I am actually using a non-spread solution to print this book (using the page shuffle off option) but am trying my best to understand why InDesign is placing the bleed on the opposite page of the PDF to better my understanding of design for print.