When a shared mailbox is added to your personal account in OWA, it does not show conversations and changing the conversation setting is not applied to the shared folder, however when opening the shared folder in a separate tab it shows conversations. This is problematic and causes issues, especially when using add-ins, as it is not possible to use add-ins on a shared mailbox in a separate tab.
It is the same issue as described in this ticket from 2017: