It seems that if a topic is started in the Feedback Hub with an Insider account, the topic is not visible to those with non-Insider accounts, they will get the message "Your account doesn't have access to this feedback".
Equally, if a topic is started in the Feedback Hub with an non-Insider account, the topic is not visible to Insider accounts.
To test this, try these two links. My guess is only one will work for you depending on what type of account you have. I can view both topics, but only by switching accounts in the Feedback Hub.
https://aka.ms/AA1aq5i
https://aka.ms/AA1aq53
Note: moderator accounts may be able to bypass the restriction.
Insider accounts are not necessarily using preview builds. Even if you sign up for a preview build for one day and revert back, the account will stay as an Insider account forever after. Therefore keeping the feedback from Insiders and non-Insiders invisible to each other seems like the wrong thing to do, as it limits the number of people who can see each topic.
So is this a bug or is it by design?