What is the reason for feeling that major ritual impurity (janabah) persists after ghusl when going to the mosque, despite this feeling disappearing when praying at home?

There is no interpretation for this situation other than it being a type of waswasah (whispers/obsessive thoughts). Therefore, one should not pay attention to it or let it be a reason to miss prayer in the mosque. It could be a trick from Satan to prevent one from going to the mosque and praying in congregation. So, one must not obey him, but rather insist on disobeying him and fulfilling the command of Allah and His Messenger by not missing the congregational prayer without a legitimate excuse.