"I have just been talking ..." does not sound idiomatic to me. The problem is the word "just" which implies a singular action, now complete. It seems more natural to use the past progressive:
I was just talking to John about this.
"Was just talking" does mean (more or less) the same thing as "I have just talked", although as usual the progressive brings a sense of concurrency, as if you are about to talk about events that were happening at the same time:
I was just talking to John about this, and he was saying that it would be better to hold off on the party until after our final exams
The present perfect progressive is used to talk about ongoing events that continue up until the present moment:
I have been talking to John about this every day, but he's still against investing any money in Bitcoin.
(Edit) Other English speakers may feel "I have just been talking" is fine, but it's not something I would say, and I would want to correct the speaker if I heard someone say it. I'm interested to hear other opinions.