#mastodon decided that your "preferred layout" was only sometimes relevant – if you choose to use the "advanced" (aka multi-column) layout, normal links to posts will now always render in the simple/single-column view because "people want to focus on the linked content".
As the simple view does not even provide a link back or a switch to the multi-column view, you're stuck until you edit the URL manually.
Here's a user script to fix this if you're also annoyed by this: https://github.com/rixx/dotfiles/blob/master/firefox/greasemonkey/mastodon-layout.js