When middle-clicking a block with the hotbar full, the replaced slot depends on the currently equiped item (block or tool). For a block, the current slot is replaced and for an ENCHANTED item the (presumably) rightmost slot not containing an enchanted item is selected. This mechanic is usefull to be able to pick bloc without having to replace items that are always on the hotbar for quick access (sword, pickaxe, bow...) BUT the problem is this only applies on enchanted items creating situation where a skeleton shows on your hardcore survival and you have no more sword on your hotbar.

Maybe this is intended as this mechanic exists since at least 1.14 but it really breaks the picking block feature in survival.