problems with names of image files in epub2

Yesterday I needed to make a small update to a book I published in mid 2021. At that time, epubcheck made a clean run with no errors.

Now I am using Sigil 1.9.20, and after modifying some small text entries in one chapter, I automatically ran epubcheck. There was a slight delay while epubcheck did its update search and downloaded and installed v5.0.0 and then I was confronted with a sizeable list of pink errors. These all concerned filenames.

The error messages are all similar to (my red coloring) :

The book has some 130 gif images, many using a naming convention like:
A+m6+M6-Ans.gif and Bb+P5-M2-m6-Ans.gif

This convention is not arbitrary. The + and - signs denote a direction from the initial letter. I would like to maintain the convention if possible.

Somewhere since I made the epub, these + signs in the names are being converted to %2B in the xhtml files and also in the opf file. The - signs remain unchanged.

The file names are shown correctly (as their original names) in Sigil's Browser pane and the book displays correctly in the readers I have tried. So maybe I could just ignore the problem but I would like to know if there is a way of preserving the file names unchanged in Sigil.

Straight editing by replacing %2B with a + sign works in the xhtml files. In the opf file, it goes back to %2B as soon as the focus is changed. The result is another fail with a different error relative to the xhtml page saying that the file name is not referenced in the opf file.