I am not sure if this is a bug or secret Easter egg.
When have the enchanting table open with nothing on the left side that can be selected (no greyed out items either) and there are xp orbs around, the book behind the xp level flashes like an xp orb.
Also at night the book apears to dim and brighten.
My set up
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Flashing
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Not flashing
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This is a bulk resolve of issues that haven't been updated to 0.16.0+ yet. Please comment below (To open the ticket) and update your affected versions. If you cannot reproduce the bug, please write in the comments so we can resolve it as "Cannot Reproduce". Thank You!