In version 1.9.3 it used to work correctly. Now, version 1.10.0, it always returns 0, while available() returns a positive value. If relevant, in my case I have a timeout set to 0. I'll post here ...

Serial.readBytes() reads characters from the serial port into a buffer. The function terminates if the determined length has been read, or it times out (see Serial.setTimeout()). Serial.readBytes() ...