/root/data/qt6/qt-host/qtbase/configure -release -make examples -no-opengl  \
  -qt-host-path /root/data/qt6/qt-x86                           \
  -extprefix /root/data/qt6/qt-target                        \
  -prefix /root/data/qt6/qt-x86                                  \
  -- -DCMAKE_TOOLCHAIN_FILE=/root/data/qt6/qt-cross/toolchain.cmake

and I use this file for cmake list ,below here,

cmake_minimum_required(VERSION 3.18)
include_guard(GLOBAL)
# Qt6HostInfo_DIR=/usr/lib/x86_64-linux-gnu/cmake/ Qt6HostInfo
# set(Qt6HostInfo_DIR /usr/lib/x86_64-linux-gnu/cmake/)
# set(Qt6HostInfo /usr/lib/x86_64-linux-gnu/cmake/)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR riscv64)

set(TARGET_SYSROOT /root/data/d1-tina/tina-d1-h/prebuilt/gcc/linux-x86/riscv/toolchain-thead-glibc/riscv64-glibc-gcc-thead_20200702/sysroot)
set(CROSS_COMPILER /root/data/d1-tina/tina-d1-h/prebuilt/gcc/linux-x86/riscv/toolchain-thead-glibc/riscv64-glibc-gcc-thead_20200702/bin)

set(CMAKE_SYSROOT ${TARGET_SYSROOT})

set(ENV{PKG_CONFIG_PATH} "")
set(ENV{PKG_CONFIG_LIBDIR} ${CMAKE_SYSROOT}/usr/lib/pkgconfig:${CMAKE_SYSROOT}/usr/share/pkgconfig)
set(ENV{PKG_CONFIG_SYSROOT_DIR} ${CMAKE_SYSROOT})

set(CMAKE_C_COMPILER ${CROSS_COMPILER}/riscv64-unknown-linux-gnu-gcc)
set(CMAKE_CXX_COMPILER ${CROSS_COMPILER}/riscv64-unknown-linux-gnu-g++)

# set(QT_COMPILER_FLAGS "-march=armv7-a -mfpu=neon -mfloat-abi=hard")
# set(QT_COMPILER_FLAGS_RELEASE "-O2 -pipe")
# set(QT_LINKER_FLAGS "-Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed")

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

include(CMakeInitializeConfigs)

function(cmake_initialize_per_config_variable _PREFIX _DOCSTRING)
  if (_PREFIX MATCHES "CMAKE_(C|CXX|ASM)_FLAGS")
    set(CMAKE_${CMAKE_MATCH_1}_FLAGS_INIT "${QT_COMPILER_FLAGS}")

    foreach (config DEBUG RELEASE MINSIZEREL RELWITHDEBINFO)
      if (DEFINED QT_COMPILER_FLAGS_${config})
        set(CMAKE_${CMAKE_MATCH_1}_FLAGS_${config}_INIT "${QT_COMPILER_FLAGS_${config}}")
      endif()
    endforeach()
  endif()

  if (_PREFIX MATCHES "CMAKE_(SHARED|MODULE|EXE)_LINKER_FLAGS")
    foreach (config SHARED MODULE EXE)
      set(CMAKE_${config}_LINKER_FLAGS_INIT "${QT_LINKER_FLAGS}")
    endforeach()
  endif()

  _cmake_initialize_per_config_variable(${ARGV})
endfunction()

,could you check about that ?

If after your "cross build" you have ended up with the .so for the wrong architecture it might generate "'/qtlib/plugins/platforms/libqlinuxfb.so' is not a Qt plugin (metadata not found)", is that a possibility?