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I’ve just realized that the WaitTime data is now defaulting to 20 ticks instead of 0, which is the cause. When summoning with WaitTime:0 the old behavior is observed. However, as this change was not explicitly documented (at least not anywhere I saw), I would still consider this a bug.

13w36a (1.7): /summon added, so this was the earliest I tested.

13w36a-14w08a (1.8): Invulnerable mobs completely damage immune, even to creative players, and fishing rod doesn't do anything.

14w10a: "Players in creative mode can now damage entities even if they are flagged invulnerable."

14w10a-15w35e (1.9): In creative, the fishing rod does "pseudo-damage", including knockback, on initial attachment, and pulling works. In survival, the bobber passes through. This is the period I believe is showing the intended behavior.

15w36a: "Most entities can now be pushed by other entities... The same class of entities are now 'magnetic' to the fishing rod bobber, and in the case of mobs/players will no longer suffer from knockback or pseudo-damage."

15w36a-present: The fishing rod can be used on Invulnerable mobs even in survival mode.

Should I make a new bug report with this information?

True, though I can’t think of a better way to find out if it’s working as intended. This seems like an oddly specific thing for a dev to have commented on before. 

I have found that it is easy to cause two villagers to claim the same job site block by rapidly breaking and placing it (can take several tries — noticed when I was trying to get librarians with specific enchanted books). However, when this is the case, they will both lose their jobs if the block is broken again.

Sometimes it can even cause more than two villagers to claim the same block — I managed to get three.