What I expected to happen was...
Mob armor spawning in the order of (most likely to least likely) Helmet; Helmet and chestplate; Helmet, chestplate and leggings; Full set.
What actually happened was...
Mob armor spawning in the order of (most likely to least likely) Boots; Boots and leggings; Boots, leggings and chestplate; Full set.
Before version 1.9, Mob armor would spawn akin to what i expected, but after this update, it started spawning akin to what actually happened.
The bug can be reproduced by simply finding or spawning zombies or skeletons in a pre-1.9 world until one spawns with a partial set of armor, and repeat in a post-1.9 world, where the bug is present, and see the difference
This may have be related to the change in command format in said update, specifically the NBT tag of ArmorItems, which as of 1.9 is designated in the order of feet, legs, chest and head, as with the way mobs spawn with armor starting in this update. This issue has been present in every version since then.
As seen in the photos, 1.8.8 mobs are seen with a helmet only, but in the other photos, (of 1.9, 1.15.2 and 20w18a) zombies and skeletons are seen spawned with armor starting from boots going up.
Edit: After a little bit more research, I am currently unsure if this is actually a bug or an intended feature.
Edit 2: I found that this issue is not present in the Bedrock Edition, possibly classifying this as a parity issue if not version exclusive. (See bedrock screenshots)