"Was just diagnosed" "Have been just diagnosed"

First of all, it is more natural to say, "I have just been diagnosed."

Basically, those are two ways of saying virtually the same thing, if it is in the context of, for example, someone saying, "When did you find out you had cancer?" and you could reply either, "I was just diagnosed," or, "I have just been diagnosed." Both mean it happened in the immediate past.

So they are the same?

Obviously, they are different verb constructions but they both sound natural and there isn't any difference in meaning in this context.

and in other contexts?

As long as "just" is included, I can't think of a context where the meaning would be different, although there may be one. But, without "just," those two verb constructions do have slightly different meanings.

For example, "I was reading about China for a report I have to do." and, "I have been reading about China for a report I have to do," have slightly different meanings about time p

Was is past simple Have is present perfect _ there must be a HUGE difference between the two.
but i dont know what the difference is exactly