The built-in line and stacked column chart shows inconsistent tooltips. The column series tooltip is missing the values from the line series (Line y-axis field(s)). The line series tooltip only shows the X-axis Line y-axis field(s), ignoring fields loaded into the Tooltip well. The tooltips should consistently show all the fields from the X-axis, both y-axis and the tooltips.
I found this forum post where Microsoft suggests this behaviour is "expected". It might be by Microsoft, the rest of the planet would likely differ.
Actually it is by design in Power BI Desktop as far as I know.
In line and stacked column chart, each column and line point is considered as a single part of the X-axis for Y-axis. The tooltip only shows the corresponding part value so you will see that when you put the mouse on the column, it shows the column value, the line value is not missing, instead, you need to put the mouse on the line point to show the value.
As a compasrion, you can enable the line marker for the visual to see the line point better.