Is "previous predecessor" grammatically incorrect?

There’s your misunderstanding. Pedants call this usage redundant. I’m saying that it’s not in fact redundant because the acronym has become a separate term. However, usage creates a form that repeats the last word of the original, causing people who don’t understand how language works to consider it redundant and therefore “wrong” or “inferior” English.

I can see a debate about whether a phrase like “the OPEC countries” is redundant.

But “an OPEC country” or a particular one like “Venezuela, an OPEC country, …” isn’t redundant nor anything else wrong or weird about it. Just like “a U.S. state”, meaning “a United States state”.

The point is that country in “OPEC country” is not redundant even to a pedant.

In at least four previous posts, you claimed that the phrase “OPEC country” is redundant. Now you say, “. . . it’s not in fact redundant . . .” and “. . . people who don’t understand how language works . . . consider it redundant.” Please explain how this is my misunderstanding.