What font value does InDesign use for Ascend/Cap height of a font? InDesign bug or font bug?

You can get some font metrics via scripting. This gets point Size, Baseline, Ascent, Descent, Cap Height, X Height

// the selected character
var s = app.activeDocument.selection[0]; 
// the fixed decimal place
var dn = 3;

if (s.constructor.name == "Character") {
    var p = getFontMetric(s).pointSize;
    var bl = getFontMetric(s).baseline;
    var a = getFontMetric(s).ascent;
    var d = getFontMetric(s).descent;
    var ch = getFontMetric(s).capHeight;
    var xh = getFontMetric(s).xHeight
    alert ("Selected Character Metrics\rPoint Size: " + p + "\rBaseline: " + bl + "\rAscent: " + a + "\rDescent: " +d  + "\rCapHeight: " +ch   + "\rXHeight: " +xh )
} else {
    alert("Please Select a Single Character")
}


/**
* Gets the selected character’s font metrics 
* @ param the character selection  
* @ return an object with pointSize, baseline, ascent,descent, xHeight, and capHeight 
*/
function getFontMetric(s){
    app.scriptPreferences.measurementUnit = MeasurementUnits.POINTS;
    var o = new Object;
    o.pointSize = s.pointSize.toFixed(dn);
    o.baseline = s.baseline.toFixed(dn);
    o.ascent = s.ascent.toFixed(dn);
    o.descent = s.descent.toFixed(dn);
    app.copy();
    var tf = app.activeDocument.textFrames.add({geometricBounds: [0,0,500,500], textFramePreferences: {firstBaselineOffset: FirstBaseline.CAP_HEIGHT, insetSpacing: 0, minimumFirstBaselineOffset: 0, verticalJustification: VerticalJustification.TOP_ALIGN}});  
    app.select([tf.parentStory.insertionPoints[0]]);    
    app.paste();
    tf.texts[0].alignToBaseline = false;
    o.capHeight = tf.texts[0].insertionPoints[0].baseline.toFixed(dn);
    tf.textFramePreferences.firstBaselineOffset = FirstBaseline.X_HEIGHT;
    o.xHeight = tf.texts[0].insertionPoints[0].baseline.toFixed(dn);
    tf.remove();
    app.select(s)
    app.scriptPreferences.userInteractionLevel = UserInteractionLevels.INTERACT_WITH_ALL;
    return o
}

Result for the selected font:

First, "ascent" refers to the amount of space designed into the font to accomodate the ascenders of the font's glyphs. An excellent definition and demostration is at https://stackoverflow.com/questions/27631736/meaning-of-top-ascent-baseline-descent-bottom-and-leading-in-androids-font

Ascent and any of the other settings that position a glyph are part of the font's metrics — the programming within the font that controls each glyph's positioning veretical and horizontal (tracking/kerning).

If you don't like what Cabin has programmed into the font, then look for another version from another vendor that might have different metrics programming. Cabin is available from Google Fonts at https://fonts.google.com/specimen/Cabin