What I expect to happen: I can spot a witch when I visit her hut (when not on peaceful), and I can fight with her.
What actually happens: the witch spawning ratio is too low (see
MC-16043 and MC-880) so you can visit her hut many times, and she will not spawn at all. But even when having her spawned from spawn egg, you will not be sure to have an occasion to fight with her.
Witch huts stay, as a rule, in shallow water. The witch (once spawned) can jump from the hut into water but she cannot go back inside. Having noticed the player, she goes toward him, even if there is a deep water between them. As a result, she drowns and there is no fight.
How to reproduce: just observe a witch hut in the game (to save your time, spawn the witch from egg).
1. To give an absolute priority for witch to spawn when only there is a witch hut (and the game is not on peaceful). As for now, the spawning ratio seems to be so low that zombies, skeletons etc. have chance to spawn but not a witch (reported many times with no solution...).
2. To add the witch a way to go back inside her hut - make the "legs" of the hut shorter, add stairs etc., or give the witch a broom to make her able to fly.
3. To add the witch some more intelligence - she really should not go to deeper water like a brainless ram but she should protect herself from drowning, especially when there is a way to go towards the player through shallow water and land.