100% black will not output to film as 100%
Came across this same issue now using CC 2017 but was able to isolate it. Customer file in CMYK color space, black at 100% nothing in the document except a 100% black rectangle. Illustrator reports 100% black but when output to film black is approximately 80%. Copying the entire content to a new document the output is at 100%. Using the original document and changing the color panel to grey scale black is 100%. Changing back to CMYK black is also 100% black. Placing the file into another dicument also produces 100% black. The latest file was created in CS6 but have had this issue since CS5. This appears to be an Illustrator bug.
Jim S commented
When the PDF was opened in Illustrator the rectangle was 100% black but film output was only 80%. Placing the PDF into another document produced 100% black as did copying the graphics and pasting into another document. In the original document changing the color panel from CMYK to grey scale and then back to CMYK the output was then 100%.
Thanks for reporting the issue,Can you please give us the exact steps that you followed while encountering this bug.