InDesign Solid 100k black become transparent when there is an object with effects

[Koko Chanel]

I noticed my 100K black become transparent when there is a drop shadow object. It shows up in exported pdf and print. The same thing happened with any object effects, inner shadow, outer glow, feather ... 

I did find a temporary solution to change the 100K to rich black and the transparency disappeared in print.

Or if I change the transparency blend space to RGB and the problem went away. I created the document as a print document and used all CMYK colors, why do I have to change the transparency blend space to RGB? 

I never noticed I have to do that before. Is there something wrong with my InDesign setting? 
Please help, thanks!

[rob day]

Hi @Koko Chanel , For print don’t set the blend space to RGB, build a CMYK rich black for large black fills. Offset inks are somewhat transparent, so if you overprint CMYK black only the colors below will show through on press. InDesign is accurately soft proofing a black overprint. The amount of ink transparency depends on the document’s CMYK profile assignment—black on an uncoated sheet will be more transparent than on a coated sheet, and the profile assignments will soft proof the appearance.