The developer suggested this article to collect a stack trace. There is a section titled “Stack traces for Flatpak apps”.
You should usually be able to find a link to report issues with an application below that application’s description in Software or on Flathub website.
I usually try running the application from terminal. While it runs, it may print messages to the terminal.
$ flatpak run org.gnome.Epiphany
Assuming the default options for an installation of Debian, the journal may provide some messages. For example, to print messages in real time, I run journalctl -f and later stop it with Ctrl+C keyboard shortcut.