Spell checking in WhatsApp Web does not work in Unify Flatpak

Hi,
I'm using Unify as a Flatpak on Debian 13 KDE Plasma (Wayland).
Spell checking works correctly in WhatsApp Web when using a normal browser, but it does not work inside Unify.
I investigated the Flatpak environment and found that Hunspell is available and working:
/usr/bin/hunspell
/usr/lib/x86_64-linux-gnu/libhunspell-1.7.so.0
The Austrian German dictionary is present:
/usr/share/runtime/locale/de/share/de/hunspell/de_AT_frami.aff
/usr/share/runtime/locale/de/share/de/hunspell/de_AT_frami.dic
And this works inside the Flatpak:
flatpak run --command=sh io.github.denysmb.unify \
-c 'hunspell -d de_AT -a'
Hunspell reports version 1.7.2.
The KDE Sonnet Hunspell plugin is also present:
/usr/lib/plugins/kf6/sonnet/sonnet_hunspell.so
The environment is:
LANG=de_AT.UTF-8
However, WhatsApp Web does not perform spell checking inside Unify, even though spell checking is enabled in WhatsApp Web itself.
Could this be related to Qt WebEngine's spell-checking support in the Flatpak?
Thanks!

Reproduce the missing spell checking in WhatsApp Web inside the Unify Flatpak on Debian 13 KDE Plasma Wayland, then inspect the Flatpak environment and Qt WebEngine spell-checking support. The report names the hunspell command, Hunspell dictionaries, and KDE Sonnet plugin as verification points; done means spell checking works in the embedded WhatsApp Web as it does in a normal browser.

Written by the indexing model from the issue text.