Kalendar was included as part of the Kontact flatpak bundle in May 2022 (kalendar#68 (closed)). Currently searching "Kalendar" doesn't return this bundle either within Discover or Flathub's search. This bundle issue is addressed by using the keyword spec (https://www.freedesktop.org/software/appstream/docs/chap-CollectionData.html#tag-ct-keywords) and adding Kalendar to the description