If I add images to titles, they are shown in preview, but missing in rendered video.

kdenlive version 20.12 - 21.04 (appImage)
os: ubuntustudio 20.10 (plasma desktop)

Ubuntu installed version (20.08.2) doesn't have this bug

Please try with the AppImage from the download page to see if this is an packaging issue.

The AppImage from download page is affected one.
Nightly build AppImage too.

I tried with a jpg and a PNG file inside a title clip and it renders fine to MP4. Which type of image and render setting do you use?

It is png image.
Rendered to MP4.

I found, if I save title first, it ask to embed image, and than, if I load that saved title, render renders images fine.