@gauravpandey thank you for all the information that’s been very useful. We have identified that this issue is due to a conflict with the ‘Conditionally display featured image on singular pages and posts‘ plugin.
When this plugin is active it stops Web Stories from saving poster images. If you add your poster image, update then refresh you will see that it has not saved. If you now disable the ‘Conditionally display featured image on singular pages and posts‘ plugin and try to add your poster image now and update it will save.
You can then refresh your site page and the images should then display. You can also then enable the plugin once more and it won’t affect the poster images you added whilst it was disabled but be aware that if you try adding or changing any other poster images in Web Stories with this active then you will face the same issue again and will need to disable before adding poster images.
I would recommend reaching out to the plugin support about this to see if they can get that fixed but in the meantime you will need to disable that plugin while adding Web Stories poster images.
Please confirm with me that this works for you and let me know if you have any further questions.