brent limitations of a sandboxed app? period?
Nope. If that were the case, your Chromium Flatpak would exhibit the same behavior. It is something with the unGoogled Chromium Flatpak.
I agree with @Axios ("By default, Flatpak downloads app-specific locale data (including translation files, locale and format settings, and dictionaries) for each app matching your system locale. So if it work when you installed flatpak should keep working ...")
I don't know what is causing this, but I'm reasonably sure that unGoogled Chromium is the place to look for a cause, not Flatpak. @Axios might be right about an unGoogled Chromium update breaking something. I can think of two ways to check this: (1) install a distro that runs unGoogled Chromium native and see what happens, or (2) install the unGoogled Chromium appimage and see what happens, in both cases over time so that unGoogled Chromium updates.