I think this is a well-known effect/"issue":
Amazon links show up as a link: MediaEmbed is not used, this is normal post parsing behaviour
Amazon links show up as embed: MediaEmbed is enabled
Amazon links don't show anything - blank space: MediaEmbed is enabled and your browser's ad blocker prevented the embed from loading.
If you want YouTube embeds but don't care for Amazon embeds, you can use this extension to only enable YouTube (or any selection of supported websites) in MediaEmbed https://discuss.flarum.org/d/31404-selective-mediaembed
As to the why the embed is blocked, I would assume it's not really an advertisement issue, but it's definitely a tracking issue. When the Amazon embed is rendered, Amazon will track that you have seen that product and might then use this to serve you targeted ads, hence why many privacy-focused browser extensions might block it.
The MediaEmbed TextFormatter plugin should definitely be improved in a way where a browser blocking the embed does not result in zero content being displayed, there should be some sort of backup link. But since it replaces the entire link with an iframe and the ad blocker removes the iframe there's nowhere to put a backup value.