Came across this when setting upright text in one column and italic text in another.
These text boxes are set with the first Baseline align to x-height. For the upright version, this works, but for the italic, it aligns to the top of the overshoot. Metrics for both are exactly the same.
Not sure what is going on?
I’ve just tested a range of other typefaces (the example above is a work in progress of mine) and some other typefaces have this same problem while others are displaying as expected. So don’t think it’s an InDesign issue.
Can you check the bounding box of your x? InDesign might literally measure the “x” height, which, in italics, might be slightly higher with the round shapes of the x (which normally is straight at the top in upright styles and thus sits flush with the x-height).
Wow — you are correct!
My italic x is rounded on the top, but if I modify the curves to end at the x-height and not the overshoot, it displays as expected in InDesign.
WTF Adobe…
Indeed, fonts should include an sxHeight metric in the OS/2 table, which defines the x-height. There may be a good reason why Adobe chose to ignore this and use their own measurement (like they do for the ascender height, which is defined by the height of the lowercase d).
At least I know it’s not an error from my end 
You can file an issue with Adobe, but good luck with that…
