I edited the bug report again to edit the version when the bug first appeared. It seems that the glitch appeared even before 18w08a. I have no access to 17w45, 17w46 and 17w47 but I still have access to 17w43. The bug isn't there in 17w43 and it is there in 17w48, so the glitch happened in the snapshots between 17w45 and 17w47, including those. If some people still have access to the snapshots I have listed, please tell me on which snapshot did MC-126595 first appear.
Confirmed for 18w16a.
Confirmed for 18w15a.
Confirmed for 18w14a and 18w14b.
Confirmed for 18w11a.
Confirmed for 18w09a, 18w10a, 18w10b, 18w10c, 18w10d.
No. I tried to reproduce the glitch on different maps and it gave the same results.
Sorry sir, but I couldn't find a way to reproduce the issue. It seems to happen randomly, or I missed something. I just can tell that it happens.
Collision is the real cause. I tried to place 400 chickens in a 1x1 spot, and in a 7x7 spot and the difference is clearly here.
Do the server has to calculate Boats and Item Frames pathfinding?
Edited the bug report again. I obtained the access to older snapshots, and it's now official that MC-126595 was here since 17w47a. Unfortunately, I have no idea what caused it.