It could be due to the format or nature of the post. Some platforms only allow specific kinds of content like photos, short videos or text in a particular style to be shared to stories. If this post doesn't fit those criteria, it can't be shared. For instance, if it's a very long and complex document that doesn't conform to the story's simplicity requirement, it won't be shareable.
There could be a few reasons. Maybe it's a technical glitch or a compatibility issue with the story-sharing feature of the other platform. Also, it could be that Spotify has not enabled this sharing option for security or legal reasons.
Yes, often they can. For instance, on Facebook, if you share someone's post, it usually shows up on their activity feed or they may receive a notification. This is a way for the platform to promote transparency and also to give credit to the original poster.