Why does pdf export sometimes create transparencies

I asked about pdf transparency problems a few days ago and accepted the answer I got: to download the most current libreoffice. Seemed to work OK. This afternoon I encountered a strange problem:
I was working on a paper with several drawings in eps format and exported the odt file to pdf; worked OK as expected. I found an error so I corrected it and the resulting pdf gave a “transparency converted to image” error. Same file with three words of text changed.
Any ideas why the error only occurred on the subsequent export? How would I find the offending image to fix it?
Thanks in advance.

Maybe if you share your file, someone can take a look and perhaps help.

Maybe if you share the file, perhaps someone can help to find the issue.

I will be happy to do that but the upload will be over 30 Mb. Instead I put some sample files on my test server at: www.iliffe.ca/adv2-c01review-2f.odt/pdf. These are old versions of the file and the pdf export works as expected. There is also /adv2-c01review-2f-ch.odt/pdf where the pdf export failed. The changes are that drawing #1-16 was moved to the other side of the page and 1 line was added to the end. The drawing was NOT changed, just re-aligned, and the text moved to the left.

Well share the writer file with the minimum number of pages to see the issue. Pdf doesn’t help except to see the export result.

I can’t create a new odt file that doesn’t have the problem. Older file was recovered from the backup and was created using a version of Libreoffice that I no longer have. (lost in Fedora update) How would I upload an odt file to this group? There are two of them posted on the test server, one before (converts properly) and one after (error converting and huge pdf file.) ONLY difference between them is that one eps file was moved from right to left using align function and added sentence.