Maybe some do, most I know don't. They can happily get along but don't rough and tumble play they way, say labs due. Not just mine, but ones that have visited or fostered just don't seem to play the way that the whippets or other visiting dogs do. Seems when talking at trials that most people's (not all of course) don't seem to play as they get older (not counting playing with puppies, I mine playing with other adults).
But just because they don't wrestle doesn't mean they don't like each other. Dekka likes Seren, but they don't play... they run around off leash together, investigating the same things and sticking together.
In short dogs don't have to play to get along. Talking to people at trials it seems its normal (not just for mine) for JRTs not to be big into playing a lot with other adults. I don't think it has anything to do with stable minded. That is like saying all people who don't enjoy rough and tumble play are unstable. I do thing they should be able to enjoy other dog's company though.
ETA Kaiden is the most stable JRT I have met. He is good with all people and great with dogs, even other intact males. He is confident and doesn't have an aggressive or shy bone in his body. Kaiden is well socialized and gets along with other dogs, however since about 3 he hasn't played with another dog.