The report does not include any file, but probably that file is using the
settings for unified colors, which are currently not available in the UI,
so it always paints black. This enables unified colors and secondary
colors for sculpt vertex colors, so it should solve that issue.

Unified color does not make much sense now as the Paint tool is the only
one that has paint capabilities, but it will do in the future when
sculpt and paint at the same time is enabled and the paint capability is
added to more tools.