Hi - I quickly found a workaround for this, but figured I may as well record it somewhere.
I'm running ArcGIS Pro 3.1.3. When creating a histogram from a raster, the "Median" statistic does not appear on the chart, even when the box is checked.
This doesn't happen with the Mean or Std. Dev. checkboxes. The problem is resolved by selecting and then deselecting the "Extent" parameter.
The rasters in question are too large to attach to this post, but I'm happy to send them along through some other means if a staff member sees this and wants to investigate. Sorry if this has already been documented and patched out - my organization hasn't upgraded us to the newest release yet.
So I'm using ArcGIS Pro 3.5.0, loaded a single band raster into a Map View and created the Histogram Chart. When I check the Median statistic checkbox, I do get a value. I scanned the internals and do not see a bug for this item you described, but I suppose it could be an unreported bug. Just guessing here, but when you upgrade it's likely you'll see the median values when the checkbox is clicked.