Looks like you hit a size limit for Lemmy’s cache. When a website uses more storage than you’d expect from most websites, you can get prompted to allow websites to grow further.

Safari prompts you after a website uses 1GB of storage. You may see this notification on other web apps as well. You should be able to retract this permission and clear the storage if your phone runs out of storage.

I’m not sure how you’ve managed to get Lemmy to use so much storage, but I can see it happen after a few weeks of browsing.