Weight Painting disappearing?

[quirozjMF7UG]

So I though this was a glitch found only when modeling, but I guess not it extends further/ I'm trying to rig this character, when I click on this and try to move it, it completely disappears, and something wrong happens to the positions. I don't know if this has been fixed, but this is especially a problem if I'm bone rigging, since I can't fix it at all.

If it happened to a vertex while modeling, I'd simply delete it and recreate the vertex, but I cant just delete the bone here. Why is this happening?

Also, why can't I edit the vertexes around the breast area? I'm trying to set it to zero, but it won't budge, it's like a darker red

[domo.spaji]

For the first: that can happen, be careful when shrinking envelopes (dont go near 0)

You can remove that bone and add it back.

Second: must be et least 2 bones influence vertex. Cant go from 1 to 0.

[quirozjMF7UG]

But why does that happen in general? Even when hard surface modeling? certain vertices just do that, and I don't know why