The ibus-gtk2 from shared-modules seems to be kind of broken. It is incompatible with ibus from gnome 3.32 runtime, which leads to various errors.

/app/lib/gtk-2.0/2.10.0/immodules/im-ibus.so: undefined symbol: ibus_input_context_set_client_commit_preedit

Loading IM context type 'ibus' failed

It is because of this update, which wasn't even tested.
d0a6cbb#diff-d777a79bcc73b6cc559d97d83520af82

Updated ibus to version 1.5.20
No additional testing has been done.

See also:
ibus/ibus@a40631e#diff-89f00c622efbd628bdc82af366f0db91

ibus from the shared-modules repo: 1.5.20
ibus from the GNOME 3.32 runtime: 1.5.17

It looks like that GIMP has already hit this problem:
flathub/org.gimp.GIMP@54d10d2

ibus from gnome runtime has 1.5.17 version. ibus-gtk2 from shared-modules has 1.5.20 version.
This creates incompatibility.
In order to fix that downgrade ibus-gtk2 from shared-modules (essentially revert latest commit from there).