By default, the Black swatch will overprint. Did you create a new black swatch or is it a rich black (anything other than 100%K default swatch)?
You can check/view these things in Acrobat.
Edit to add a screen shot. Both the black rectangles are 100% K. The top one is the default 100% K swatch. The bottom one is a new swatch, still 100% K.
Not heard of it nor experienced it, but that's a concerning development. Any chance your client can submit their issue directly to the forum so that we can try and replicate the fault? Have you tried to replicate the fault on your own machine, or attempted to open the client's indesign file to check for issues?
Even if the black was set to overprint or knockout wouldn't make a difference here. It's the pink text that's causing the issue.
Some things to check
1) Make sure that for the text frame under Window>Output>Attributes that the the text frame and text is set to overprint.
2) Make sure the pink isn't a spot colour (not sure why that would make a difference perhaps if it's a tint of a spot?)
3) Make sure the client hasn't changed the paper colour to Pink and tried to apply that to the text. Make sure it's a new swatch set to a pink colour that's not spot or set to overprint anywhere.
You could also ask your client to use Window>output>separations preview to see where the problem lies within InDesign.
Black over printing in the example I show is definitely an issue. My pink texts are not over printing in the screen shot. But in the top example it will, by default, be over printed by the black. Assuming that the black is printed last.
Look at the PDF at the link in Acrobat. Top is the default black swatch. Middle is the new black swatch. Bottom is the default black swatch with the pink text set to overprint per the #1 suggestion.
I've read this now a few times, and I'm no longer sure which thing is supposed to be set to knock out and which to overprint, and which is changing. Seems unlikely to me that anyone would deliberately set the pink to overprint on black.
I have asked the Customer to "Package" the job so we can take a look at it. I agree Eugene the issue is not with the background black which is 100% K. The pink text on top is simply 100% magenta. There is other green text on this same black box that has worked just fine (knockout). In another location there is orange text (50 M & 100 Y) that is also overprinting which they say was set to knockout just like the pink text in the native application. They think this is happening when they export the PDF. I have never seen this happen nor heard of it happening and we deal with lots of PDF file on a daily basis.
No answers until I get the "Package". Thanks everyone!
But in the top example it will, by default, be over printed by the black. Assuming that the black is printed last.
In your example I don't see how the ink order would matter (besides offset inks are transparent), if you look at just the black plate it shows the pink text is knocked out. Overprint only affects objects underneath the object set to overprint—the pink text is on top of the black and it is set to knockout the black below.
I know that, Rob.
Go to color warnings. Turn on the overprint warning. Now look at the top and middle examples. Thing is that most all of the rips I've been involved with have 100% k set to overprint and the default black swatch in ID, though it appears to knock out, can be overridden by the rip without intervention by pre-press.
It will not happen with the middle example. So I've just created my own black swatch in this type of usage.
With the further info from the OP, it doesn't sound like the issue is the same, though.
But the complaint is the pink object's overprint setting and not the black below. Acrobat does correctly show the overprint value of your black boxes, but whether it's true or false doesn't matter because there's nothing for them to overprint.