Inner shadow bug when exporting to PDF

Dear all, 

I've adapted a typeface to be able to have the overlapping letters in indesign. 
To do this i have hardcoded the white (fill) and black (outline) into the font in glyphs. Then i found a way to color this font. I use an inner shadow and just offset the shadow to the maximum allowed distance. All was going well up until the time that i exported the file to pdf. 

On the screenshot you can see the bug on the right hand side, the letters get pushed a random distance to the right, i tried different effects but it all seems to have the same result when i export. 

Any thoughts? 

Kind regards.