I have to ask the basic question: Is V- properly connected between the Null Pixel (NP) and the rest of the string? What you are describing would cause me to look at a few things:
V- from NP to rest of string.
Data from NP to rest of string
NP Output section or first pixel Input section has gone bad. Since they are far apart I would replace the NP first.
You did not mention any connectors between the NP and the first pixel. If there is one it may be having an issue.
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