Is this just a display problem or does it affect deformations as well? Does it happen with all envelopes?
I'd lean towards it being a display problem because even if you edited those vertices manually in some form (entering a weight, exclusion, applying some solver), which would exclude them from being influenced by the envelope, or changed the transition curve, you'd see some falloff between the vertices, unless there is either an actual split or double vertices. Only exception I can think of would be a face selection lower in the stack.
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I'd lean towards it being a display problem because even if you edited those vertices manually in some form (entering a weight, exclusion, applying some solver), which would exclude them from being influenced by the envelope, or changed the transition curve, you'd see some falloff between the vertices, unless there is either an actual split or double vertices. Only exception I can think of would be a face selection lower in the stack.