I posted this in the gameplay forum and someone suggest I post it here. Every time I've popped my Well in PoH there's been some weird healing delay at random times. So much so, that cursed thralls are killing me while I have 70% of my health and regular thralls have chipped my health off without it ever recovering. Here are two videos of it randomly pausing it's healing while I'm inside the Well.
https://m.youtube.com/watch?v=N76Ew8rvBbM
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It does heal you, you are standing in a fire and taking other damage. Its not like you are unkillable in a well. You can also see that it is healing you since your health goes instantly up on several occasions.