The "pick block" function can switch to a hotbar slot with an item in it, when:

• the hotbar is full and
• all previous slots (from the selected slot on) contain enchanted items.

Example to reproduce:

• Arrange items in the hotbar as follows: enchanted pickaxe, ench. shovel, ench. axe, ench. bow, ench. sword, in Creative mode enchanted stick, cooked chicken, water bucket, torches.
• Have netherrack in the inventory and a block of it nearby.
• Select slot 1.
• Use pick block on the netherrack.
  Expected result: Pickaxe and netherrack get switched out (netherrack to slot 1, pickaxe to wherever the netherrack was in the inventory before).
  Actual result: Slot 7 (chicken) gets selected and switched out with the netherrack (netherrack to slot 7, chicken to netherrack spot).

This was very likely introduced when MC-51984 got fixed.
Possible fixes, should be discussed: Change "switching immunity" from enchanted items to tools, maybe items with a durability bar if that's easier to handle in future versions. Or remove immunity completely.