SOLVED.
Easy(*) as duck soup:
Just install the matching version of gnome-calendar from flathub instead of the most current one.
* It involves a bit of fiddling though, by having to first install the latest version and then “downgrade” it to the commit that matches the desired gnome version. And then, of course, freezing that downgraded version from being almost instantly auto-updated to the recent version using flatpak mask.
A bit more info on downgrading here:
And here for flatpak’s mask command:
https://man7.org/linux/man-pages/man1/flatpak-mask.1.html
Cheers, Thomas