I usually tend to think this type of error is caused by exposure to sun or heat causing a color change in the paper or glue.

BUT, there is one other possible explanation that would allow just a few of this error rather than a whole sheet. When you are close to running out of adding machine tape, you can tell because of a pink stripe that begins to show. Perhaps the paper the inlays are printed on have a yellow stripe to show the end of the roll is near and this inlay was printed on top of that yellow stripe?

Perhaps you could call someone at Paulson and get an idea of what they think might have cause it?

Greg