There seems to be a bug with what the black hover info tooltip does when a scatterplot chart has multiple series. With just 1 series of data it works fine, as expected.
With multiple series it darts around randomly, although it shows the same value. All additional data series have
hoverinfo: "none"
where the original series has
hoverinfo: "y"
With the plot having
hovermode: "y"
(broken version)
EDIT:
I have found that a workaround is to change hoverinfo: “none” to hoverinfo: “skip”, but to me this seems like a workaround rather than a solution.