Hmmm, something has gone wrong. When you view communities without parameters in URL, the buttons that let you choose have disappeared.
Compare:
https://piefed.social/c/[email protected] with
https://piefed.social/c/[email protected]?layout=list

Edit: it doesn't seem related to URL parameters actually. The plot thickens.
Edit 2: fixed upstream in the code now; the change will make its way to the website in a day or so. Thanks for bringing this problem to my attention.

I've set those 2 communities to default to 'gallery mode', and I'll look into where the buttons have gone (although they only change the temporary view, not the default)

Looks like you need to use "add remote". I've added this the Album Art one and it does Tiled mode by default :
https://piefed.social/c/[email protected]

Try for yourself with the Film posters one :
Go here : https://piefed.social/communities Then click on Add remote -> https://piefed.social/community/add_remote and then search for ![email protected] and add it.