Sorry for the delay in response to your query and apologize for the inconvenience caused.
Let us know how this file was converted into PDF and is it possible to share the file with us so that we can try to replicate the issue at our end, to share the file refer to How to share a document Link: https://forums.adobe.com/docs/DOC-7161
Is any transparency effect was used in this PDF file?
If the file was flattened , then the possibility is that the transparency effect would be ignored by the process.
Hope this helps, and keep us posted with the results.