I have a shiny new 320 plus v4 (4.16) and it seems to work fine. I get a background reading in the 20s-30s and it seems well behaved.
However I have an old 1950s watch and the front of that reads about 460 counts per minute or 3uSv/h and the back 190 / 1.25 which is impressive.
I don't understand this as I would assume luminous paint would mostly give off Alpha and even it there was a lot of Beta how's it going out the back (it's a stainless steel case)? I do understand the tube is fairly long in the device, but still.
It's true that natural Radium emits mostly Alpha particles, but there are many different isotopes, and almost all particle emissions are accompanied by gamma rays. This must be mostly what you read from the back. There may be a portion that is secondary Gamma radiation, where an Alpha or Beta from the Radium hits another material, and that material in turn emits a Gamma ray.