This is a recent post and the screen capture below is what I see. I don't use a Mastodon client. I only access it from the C.im Website, if that matters.
I also don't use the Gutenberg editor in WordPress. I use the Classic Editor (in case that matters).
And this post was published on a multisite network (using subdomains).
The "Not Available" is where the Featured Image should be.
https://c.im/@michael@middle-earth.xenite.org/111063510798864523