@simon thank you again! I added very less custom css and long about page description, which I removed but still it was showing issue.
I rebuild app after this and found there is new discourse version. Now I see category banner is showing properly. I added back all plugins, custom css code and everything is working fine now.
Issue must be somewhere but not able to catch this time because it is not showing with latest version. If I see again then will update this thread.
Thank you very much for your help. highly appreciate!