Reply from Roon support:
What likely happened here is that Debian upgrades often replace older versions of the libicu library with a newer one, and only install it if some system package explicitly requires it. For example, Debian 12 included libicu72, while Debian 13 provides libicu76. During the upgrade the old package is removed, but the new one may not be installed automatically if no default system component depends on it.
That’s why it looked like the package “disappeared” after the upgrade, and Roon couldn’t start until you manually installed the newer version.
Torben