Question resolved.
The describing class did not take into account the byte order. Foolishly I did not consider the byte-order to be an issue within the bits in the byte itself. But this is obviously the case:
Example-byte: 0xf7=1111 01 11 (Big Endian: MSB first)
@BoundNumber(size="4", byteOrder=ByteOrder.BigEndian) //Corresponds to 1111
private byte b1_3_notUsed;
@BoundNumber(size="2", byteOrder=ByteOrder.BigEndian) //Corresponds to 01
private ParkingBrakeSwitch parkingBrakeSwitch;
@BoundNumber(size="2", byteOrder=ByteOrder.BigEndian) //Corresponds to 11
private byte b1_1_notUsed;
Now encoding returns 0xf7=1111 01 11 again when it has been re-encoded.