I don't know if the reverse is a new thing or if it's always been like that.
Here's a comment in a lemmy issue describing mastodon images not appearing in lemmy from last year. Since the issue was closed three days after 0.19.3 released I had expected it to work in the next version 0.19.4, but it appears that was not the case.
Edit: While I'm here, I noticed piefed also doesn't support it, so was considering opening an issue to support attachments, but I imagine there wouldn't be much desire to do so until mastodon integration is more of a real thing? though I think other software does it, like Plemora/Akkoma/Iceshrimp, I didn't check but did see the code for it being discussed for plemora at least