Средства и методы разработки

Наверное, глюк. Но это скользкая тема. По ссылке хорошо сказано -> https://gcc.gnu.org/onlinedocs/gcc/Volatiles.html

As bit-fields are not individually addressable, volatile bit-fields may be implicitly read when written to, or when adjacent bit-fields are accessed. Bit-field operations may be optimized such that adjacent bit-fields are only partially accessed, if they straddle a storage unit boundary. For these reasons it is unwise to use volatile bit-fields to access hardware.