Building KDE RPMs locally, any package that runs /usr/libexec/qt6/uic, e.g. kate, I see:

Detected locale "LC_CTYPE=C;LC_NUMERIC=en_AU.UTF-8;LC_TIME=en_AU.UTF-8;LC_COLLATE=en_GB.UTF-8;LC_MONETARY=C.UTF-8;LC_MESSAGES=C;LC_PAPER=C;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=C.UTF-8;LC_IDENTIFICATION=C" with character encoding "ANSI_X3.4-1968", which is not UTF-8.
Qt depends on a UTF-8 locale, and has switched to "C.UTF-8" instead.
If this causes problems, reconfigure your locale. See the locale(1) manual
for more information.

This seems to be caused by /usr/lib/rpm/macros, which has:
LANG=C \
export LANG \

replacing C with C.utf8 solves the issue.

(Sorry if this should have been filed against RPM upstream.)