Fiona T

No problemo. I didn't think about the extra stuff until you commented, so thank you as well.

Ok I've attached a screenshot of me duplicating the problem. I went 90 blocks (which is about 5 chunks) from my house at night, let mobs spawn, then went back to my house. Slept through the night and then waiting until "noon" to go back to that area. The mobs didn't start burning or despawning until I could see them.
This does seem to be related to render distance in the video settings but again, mobs are supposed to automatically despawn when out of chunk range, instead of just freezing.

Yeah that's the thing though; it's out in the open plains in daylight, no trees or anything. And it's all the mobs that spawned, not just a few of them. I might try increasing my render distance to see if that helps, as I have it consistently at 5 chunks. I don't know if that makes a difference or not.
Another way to explain the issue is by comparing it to the Enderdragon freezing when it gets too far from the player. This happens with monsters but they don't despawn at that point, they just freeze until I go within range again.
The main reason this seems to be a problem to me is because I never saw it happen until about version 1.6. Before that monsters would automatically despawn when the player went out a certain distance. According to the wiki that's what's supposed to happen as well, but I'm just not seeing it now. :/