I’m from California and I had an Irish colleague there who, despite otherwise being amazingly well integrated, stuck to a hard “t” for certain cases of “th.” S

I’m from California and I had an Irish colleague there who, despite otherwise being amazingly well integrated, stuck to a hard “t” for certain cases of “th.” So for instance if I might say “These three things,” he would say “these tree things.” He was clearly able to make the sounds but in some cases he couldn’t. I never felt comfortable asking about it because I thought it might be a class thing.