Shape Key Inconsistencies

So I’ve been having this problem for ages and can’t figure it out.
It’s a little difficult to explain also, so hopefully I can get it across ok;

Basically, it involves going back to edit the base shape of a model that contains a lot of shape keys (a character model, in this instance)
The issue is with how well this does or does not work.
Sometimes when I go back and edit the base shape, the changes automatically update all of the other shape keys and everything works great - activating any of the keys will also take the base changes into account.
Other times, seemingly without doing anything differently (that I can tell), the changes won’t update the rest of the keys - and activating any of those saved keys just reverts the changes made.

This is not something that can be solved with the ‘Propagate to shapes’ feature, as that just undoes the pre-existing shape key’s effects. Also, it doesn’t involve changing the vertex count of the model - I’m only repositioning existing vertices.

I’m using 2.81 at the moment, but this has been a problem since I first discovered shape keys - at least as far back as mid-2.7x, so if anyone can help and tell what may be happening I would greatly appreciate it~

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I’ve seen similar inconsistencies in dinking around w/Shape Keys, mostly about adding Subd modifiers to them. It’s hard to characterize but list operations seem to work differently when a Subd modifier is added.