Messages by remote Pleroma users are missing from profile previews, and the local feed?

When locally previewing the messages posted by people on remote Pleroma and Mastodon instances, some of their messages don't show up. When following a Mastodon user, their messages did show up in the feed (/main/friends). However I noticed that some messages by a less active remote Pleroma user I followed were missing from my feed.

To reproduce:

1. Find an active Pleroma user
2. Then visit another instance on the fediverse and search for the user with the button at the top of the page. Click to view their profile.
3. Then compare the recent messages on that page with the messages listed on the profile of their home server. (Click on the square and arrow icon in the top right of their local profile mirror so you see their profile from the perspective of the server that hosts them.)
4. Follow the user and then compare their message history in your local timeline (/main/friends) with the profile on their home server.

Some messages will likely be missing from the local mirror / user preview. Some messages will also likely be missing from your local feed as well, including original messages, re-posts and replies.