If bob makes a private post (ie, followers only), only the instances of people he follows will recieve the post. The instance will see that its supposed to be private, and not show it to everyone.
This may, gotosocial.example, mastodon.example and pixelfed.example have the post, but don't show it. misskey.example won't have the post.
Then, if gotosocial.example (hypothetically) had a bug where it ignored posts visibility settings, those posts would be shown, since the post is sent to that server.
If misskey.example had a similar bug, nothing would happen as the post wouldn't have reached that server anyway.
private posts are only sent to instances that either your followers or the list of people you want to see the post are on. If they all co-operate, you will be fine.
No, Imagine this
There is @bob@pixelfed.example their is their friend, @joe@mastodon.example. bob also follows @jane@gotosocial.example
If bob makes a private post (ie, followers only), only the instances of people he follows will recieve the post. The instance will see that its supposed to be private, and not show it to everyone.
This may, gotosocial.example, mastodon.example and pixelfed.example have the post, but don't show it. misskey.example won't have the post.
Then, if gotosocial.example (hypothetically) had a bug where it ignored posts visibility settings, those posts would be shown, since the post is sent to that server. If misskey.example had a similar bug, nothing would happen as the post wouldn't have reached that server anyway.