Can you provide a full test case? Including a couple of random lines of the code is not enough. This issue and the stackoverflow question are far from clear.

Use case: create a single deck by merging all the slides from different decks

Source decks has different slide layouts, sizes.
To load decks source decks, and merge them I'm using Apache POI 5.2.3. 

0) create an empty target deck
1) load decks from input stream
2) iterate over slides on the deck
4) create new slide in the target deck using the layout (each slide can have different layout), contents of the source deck
5) repeat step#1 until all the source decks have been imported to target deck as new slides
6) export the merged deck to file


Each step has inbuilt functions provided by Apache POI which have been mentioned in stackoverflow as well.

Finally, while exporting the target deck using write operation of class:
XMLSlideShow, it throws mentioned exception.


  Note: this exception is only logged if we use slide layouts of the source slides to create target.

  Code for each step has been mentioned in stackoverflow otherwise I can post it here

That is not what I asked for. I asked for all the code in a runnable form. There is a likely reason that noone has commented or answered on your stackoverflow question: you have not provided a reproducible example.

https://www.tutorialspoint.com/apache_poi_ppt/apache_poi_ppt_merging.htm

The code is similar to the mentioned reference.

for(XSLFSlide srcSlide : src.getSlides()) {
         
            //merging the contents
            ppt.createSlide().importContent(srcSlide);
         }

This requires improvement. If you would see the bottom of the reference, would only find content but source layouts

Fair enough if you don't want to right a test. I don't want to write one for you. Maybe someone else will.

I've already shared most of things, I'm not sure what else you are looking for?

Only change in my codebase is source of input stream. Otherwise the simplest possible Java code is available on the reference URL. We can do a google meet if required

Please use the reference project - https://github.com/Jatish-Khanna/apache-poi-example.git

I don't have much time to spend on POI right now. One thing to try yourself is to try out the `importContent` methods on XSLFSlide, XSLFSlideLayout and XSLFSlideMaster. These are the methods used in our tests when copying slides and/or their layouts to other slideshows.

One of the POI unit tests does this.

            XSLFSlide srcSlide = srcPptx.getSlides().get(0);
            XSLFSlide newSlide = newPptx.createSlide();

            XSLFSlideLayout srcSlideLayout = srcSlide.getSlideLayout();
            XSLFSlideLayout newSlideLayout = newSlide.getSlideLayout();
            newSlideLayout.importContent(srcSlideLayout);

            XSLFSlideMaster srcSlideMaster = srcSlide.getSlideMaster();
            XSLFSlideMaster newSlideMaster = newSlide.getSlideMaster();
            newSlideMaster.importContent(srcSlideMaster);

            newSlide.importContent(srcSlide);

I've already reverse engineered and tried multiple approaches including the suggested one. Please check the stackoverflow post for additional details.

 - POI doesn't offer extension
 - when trying to create a slide from layout, it overrides instead of adding a new layout

Do you need any other information to solve the bug?