distortion of portrait format pictures

Hi 

Welcome to the community! We're sorry to hear about this. Please answer a few questions below-

1. Could you please let us know in which format did you export the file? 
2. Did the slideshow appear correctly in the preview before exporting, or did the rotation only happen after export? 
3. Have you tried re-exporting the project to see if the issue persists?
4. Did you try using the other suggested browsers (Safari, Microsoft Edge, and Google Chrome) or incognito mode to see if it makes a difference? 

Also, try clearing the browser's cache and cookies, and restarting the browser and try again. For more information on how to clear cache and cookies check this link- https://adobe.ly/4a5ipUF and how to use incognito mode- https://adobe.ly/4a2UzZV 

Let us know if that helps.

Regards,

Tarun