Equipment dropped from zombie conversion to drowned drops in unpredictable locations

When zombies convert to drowneds in 1x1 spaces with a hole for a player to punch them (such as in most xp farm designs), the tools and armor (equipment) they drop will be "launched" out in odd and seemingly random directions, even if a hopper is placed underneath the zombies. Closing the this hole prevents the items from flying out, but this obviously makes xp farms impractical.

How to reproduce:

1. (fig. 1) Build a 1x1 space enclosed by blocks on all 4 sides. Build this at least 4 blocks high.

2. (fig. 2) Punch out 1 side block on one side only. Place a block at the bottom of the chamber and a trapdoor to block baby zombies from escaping.

3. (fig. 3) Spawn zombies into the chamber.

4. (fig. 4) Submerge the trapped zombies to begin the conversion process into drowneds.

 Results
(see video clip from YouTube link) Watch as the tools and armor the zombies drop when they convert into drowneds are launched out in seemingly random directions.

Video clip

Expected Result
Items dropped from these zombies upon conversion fall down instead of launching out in directions as seen in the video clip.