You should assume that whatever you upload to the internet without encryption is public. Bereal was always advertised as a tool for sharing your life with others, so this isn’t unexpected for me. The only misleading part is that if you share with friends only, it’s not just your friends, but also the service provider, so you should assume that it may leak. I never used bereal so I’m not familiar with its features, some of my friends use it and I just heard about it from them. From their description it didn’t sound as an app where you should expect privacy.
Is Lemmy a privacy nightmare? No, because you know that everything is public here, admins can even see your DMs.
Do not share private data on the public internet
Its original idea is actually interesting. Influencers always share perfectly composed scenes of their life, so it seems like they are always traveling, always doing something interesting. This app asks you at random times to please share what you are actually doing at that moment, it sends a notification when you should share, it has a window of some minutes to take a photo. You never know when the next bereal notification will show up. You can be sure, if someone always shares something interesting from their life constantly is not just faking it, it’s actually real.
I never used it, but some of my friends do, so this is based on their explanations.