Okay, I took file /proc/config.gz from the running system, unzipped it and compared the files. Here’s the difference:
***** config
#
# Compiler: arm-linux-gnueabihf-gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
#
***** DEFCONFIG.TXT
#
# Compiler: arm-linux-gnueabi-gcc (GCC) 8.5.0
#
*****
***** config
CONFIG_CC_IS_GCC=y
CONFIG_GCC_VERSION=90400
CONFIG_CLANG_VERSION=0
***** DEFCONFIG.TXT
CONFIG_CC_IS_GCC=y
CONFIG_GCC_VERSION=80500
CONFIG_CLANG_VERSION=0
*****
***** config
CONFIG_ARCH_HAS_GCOV_PROFILE_ALL=y
CONFIG_PLUGIN_HOSTCC=""
CONFIG_HAVE_GCC_PLUGINS=y
CONFIG_RT_MUTEXES=y
***** DEFCONFIG.TXT
CONFIG_ARCH_HAS_GCOV_PROFILE_ALL=y
CONFIG_PLUGIN_HOSTCC="g++"
CONFIG_HAVE_GCC_PLUGINS=y
# CONFIG_GCC_PLUGINS is not set
CONFIG_RT_MUTEXES=y
*****
Not many differences. Could the GCC version be the root cause?.. I built the kernel on Ubuntu 20.04, where GCC 9 is installed by default.