For simplicity, let's say two charts (a candlestick chart and a bar chart) have the same data for the x axis. When there are null/undefined values in the y axis data and hover state is shared between charts using Fx.hover, the pointNumber value taken from the plotly_hover event doesn't correspond to the correct element in the candlestick chart - the position is offset exactly by the number of null values preceding the hovered element - while the bar charts treats null values correctly and the correct value is hovered.
See the codepan example demonstrating the described behavior: https://codepen.io/jakubkottnauer/pen/xQQYje