@evan We don't know visibility of Bob's reply.

@skobkin the question is about what that visibility should be.

@skobkin the question is about how it should work.

@evan
As I said in my reply in another branch of this thread, different people may have different expectations.

For me it should be visible for subscribers of both of them. But it's technically difficult and most likely wouldn't be implemented.

Why? Because otherwise subscribers of both of them would see only half of the thread without being able to grasp it in it's entirety.
Especially considering that most of the AP software I know doesn't allow to extend visibility level from what it was in the OP. So making it a bit more loose by default would be a sane choice to maintain consistency of threads between different instances.

@skobkin as someone who posts a daily poll on the Fediverse, I am aware that different people have different opinions on a variety of topics. That's what polls are for.

Thanks for sharing your opinion. With that option, the number of people who can read replies gets smaller and smaller as the conversation goes on. It makes followers-only posts really hard to use.