The version is actually 18.04.2 according the "About kdenlive." Adding a slideshow clip using the file name pattern does not play all the images found. Add a slideshow clip to a project. Change from MEME to Filename Pattern for selection. The first file fills in and several images files with the same name pattern fill in the file box. In my case, 6 files. It is added to the clips. Select the clip and play back in the clip monitor. Only the first file plays back. I was using jpg files.
This bug is outdated, all the timeline code was rewritten to avoid exactly this kind of issue. This is fixed with the current version. Feel free to reopen this bug or create a new report if this is still happening with the latest version (https://kdenlive.org/en/download/)