Thank you for pointing to a post which might explain similar issues. However, the issue which I had doesn't come from the attempt to produce a clip in protrait mode from a video in landscape mode. In my case, I want to produce the clip in the same protrait mode from a video also in protrait mode, while maintaining the same resolution.
In the above mentioned post, there is information about producing a video with 90deg rotation flag. I don't really know how apply or edit rotation flag in PD, as I couldn't find any information regarding "rotation flag" in PD's help secttion.
Following is the quote from the post mentioned above:
"However, there are 2 basic ways of producing a 9:16 video for display.
Using 1080x1920 HD as an example :
Produce a 1080x1920 video with no rotation flag
or
Produce a 1920x1080 video with a 90deg rotation flag.
Both, no doubt, have their pros and cons but I understand that producing a 1920x1080 video with a 90deg flag has better compatibility with display hardware and software. I can't really comment on that, above my technical level!!"