You really don't need an account on every Lemmy server. If you just pick one that meets your needed values, you can access all the other federated Lemmy content on other instances.
The only reason I can think of to maybe do this is to have an SFW identity and an NSFW identity, and then you would just have your SFW on an instance that blocks NSFW, and your NSFW on an instance that does not.
Your identity is tied to your instance for example I am @[email protected] and anyone who interacts with me from another instance can see that. And if someone chooses the same name at another instance they are a different identity, and the two don't really cross-pollinate, just make sure to include your instance with your username whenever you share it.
Yeah. One thing I'm not really liking about Lemmy so far is the NSFW posts/avatars. The blur isn't blurry enough in the images and you can see what's behind it, and the avatars are a free-for-all. Not great if your kid if peaking at your screen while you scroll.
Sounds to me like the server had their caching misconfigured, resulting in a personalised page being put in a public cache. Normally only static resources such as images and static webpages would be put in a public cache.
Currently I don't think there's any private information that could be leaked this way in lemmy, but it might be wise to notify the instance admins about this.
I wonder if it's the same problem I was having yesterday when I opened a bunch of communities in tabs and subscribed to them, and some of the tabs showed the wrong info in the box. (probably also caching)