I'm still on 2010; is Inventor still struggling in Remedial Trig?
The part doesn't really matter as it's a fairly common occurrence; just felt like finally venting about it. When constructing the sketch (any sketch), applying an acute angle displays correctly. Complete the sketch, go on with part building, come back to edit the sketch; the angular dimension has about a 50-50 chance of displaying as the complement (always the proper value, so that's something). Drag it around; doesn't change. Surely I'm not the only one who's encountered this?
Make sure your sketch is fully constrained, I have not seen this behavior, But I started with inventor 2012. Maybe it has been fixed. I have had my sketches "blow up" on me though.