Yeah they compile to the same machine code operations though. If those machine code operations aren't right as a bitfield then they aren't going to be right done manually either.
Bit field support being buggy exactly means that they don’t compile to the same machine code as the bit shifting/masking code you would write by hand (if your hand-written code is correct).
ARM is little-endian, and by tradition bitfield bit indexes are assigned from least significant (bit 0 in ARM terms) to more significant. b occupies bits 4-7 inclusive.
