Secondary x-axis numberline doesn't show on histogram plots

I'm trying to make a plot that histograms two variables (min, max) for two sources (two different LEDs).
The plot should have two y axes and two x-axes; both sets of minimum values are plotted with a given pair of x-y axes, and both sets of maximum values on the other. This appears to be straightforward to implement; just specify the two additional axes in the layout and set the corresponding traces to use them, but for some reason although the secondary x-axis title is shown at the top (as desired), the corresponding axis numberline isn't.
reproducible example
Apologies for the somewhat contrived example code, it's hacked out of a wider webpage, but i think is easy enough to understand.

从链接的 JSBin 可复现示例开始，确认次级 x 轴的标题会渲染，但其 numberline 不会渲染。跟踪相关的 plotly.js 轴渲染路径，并使用直方图配置验证该行为；当次级 x 轴的 numberline 与其标题一同出现时，即表示完成。