There is a bug in C++ code generator.
I attach simple XML schema test.zip to reproduce the bug.
Command to generate C++ code is:
$ java -Dsbe.target.language=Cpp -Dsbe.output.dir=./test_src -Dsbe.errorLog=yes -jar sbe-all-1.21.0.jar ./test.xml

There is throw std::runtime_error("buffer too short for flyweight [[E107]") in the GroupSize constructor. So an exception of type std::runtime_error may be thrown but the throw list noexcept doesn't allow it to be thrown. This will cause a call to unexpected() which usually calls terminate(). You should delete SBE_NOEXCEPT.

test.zip

inline std::uint64_t resetCountToIndex() SBE_NOEXCEPT
{
    m_count = m_index; 
    GroupSize dimensions(m_buffer, m_initialPosition, m_bufferLength, m_actingVersion);
    dimensions.numInGroup(static_cast<std::uint8_t>(m_count));
    return m_count;
}