In a recent exploration of the mine, I noticed there were zombies spawning, and so decided I would try to figure out a way to contain them. I settled on putting in 'hatches'; I built walls at choke points to stop them from running up behind me and swatting me as I worked. After going to a corner of one room with ore near the top and mining it down, I noticed I kept getting proximity warnings, as if zombies were approaching. They would sort of pulse on and off, and I didn't see anything when peeking through the doors I had placed, so I didn't think much of it... until I started mining a new area.
As I broke through the rock, an entire horde of zombies came pouring out. At least 8 or more, all clumped together in a big pile. I backed off, running through one of my doors, but they were still there, moving as one. I tried to let one through and close the door so I could thin them and they rushed through. I was forced to leave the mine entirely.
Eventually I set up to die; placed a tent and bound myself to it so I could revive right there, built walls, ate food and generally got ready to make any number of suicide runs into the mine to retrieve all the ore I had left behind, but when I went in, the zombies had all despawned.
I don't think this was meant to happen. I think it was only supposed to be one, but because I was in a certain 'sweet spot' it repeatedly spawned one after another until there was a massive group of them in that small area, that I then released when I dug into it. And although I survived it (barely) and they despawned once I left, that would have made that mine almost inaccessible without being forced to die over and over to clear the zombies, with the chance they would just be spawning more inside at the same time.
I feel this needs some looking into.