To be quite fair most of them are just memorising it. They do take classes to learn the language but their conversational skills are really basic and their pronunciation is distinctly Korean.
This is what happens in most of the groups. Unless they stay base in Japan, they will not learn a fluent level of Japanese.
Most Soshi struggles, expect Sooyoung who was in Japan for some years, and perfectionist Seohyun, if she has to learn something, she will do it.