I'm always slightly afraid when things are inconsistent.
That "-p" output looks right when it did work. Could you try creating a serial log a few times [--serial-log <file>], and see what you get when it works, and when it doesn't?
What exactly is the device that you have? I've never tested with anything that wasn't an ELM327. I've also never seen anything create a /dev/ttyACM device before. Where did you get it from?
My actual suspicion is that it's just a cheap chinese device that may just be not-entirely-functional [which wouldn't be unprecedented]. Are you able to open it and look inside? How is the main chip inside labelled? Does the circuit board look like it's properly soldered?