Using the following Dockerfile...

FROM alpine:latest AS build

# Install build dependencies
RUN apk add --no-cache \
 qt6-qtbase-dev \
 qt6-qtdeclarative-dev \
 g++ \
 make \
 git

WORKDIR /app/QJsonSerializer
RUN git clone --depth 1 https://github.com/Znurre/QJsonSerializer.git . \
 && /usr/lib/qt6/bin/qmake \
 && make -j`nproc` \
 && make install

...with the following command (on x86_64):

docker buildx build --platform=linux/arm64/8 .

...will result in the following error:

#0 1.952 /usr/lib/qt6/mkspecs/features/toolchain.prf:76: Variable QMAKE_CXX.COMPILER_MACROS is not defined. 
#0 1.958 Project ERROR: failed to parse default search paths from compiler output

It works good when not using docker buildx. Tested with both Alpine, Ubuntu and ArchLinux ARM. As soon as version of Qt is >= 6.2, qmake will always fail with this error when docker buildx is used.

It worked good with 6.1.

Attached qmake debug output when run in the buildx environment.