That's not exactly what I getting at but you do raise an interesting option there.
I suppose it could well be possible that FF/CB are trying to prevent details that he wrote in his letters to MWT from being made public. I would have assumed that FF was aware of what CB wrote to MWT and Jutta Rabe but perhaps he wasn't and they are now fearful it could hinder any future defence case if the details he divulged are made part of the public record. Perhaps they have requested them to be retracted and are threatening legal action if they broadcast details of what CB has written.
Details for the 3rd programme were not listed on the German TV guide, but this is what it was said to cover according to the Production team's synopsis-
EPISODE 3: TESTING THE ALIBI
Christian B gives Mark a possible alibi revealing that he had a holiday fling with a German 18 year old girl the week of Madeleine’s disappearance. Mark tests this by tracking her down and speaking to her now husband. He also tests the German authorities evidence further by examining the witness to an alleged confession made by Christian B and the significance of his vehicle being re-registered to a new owner the day after Madeleine disappeared. Christian writes to Mark with a detailed account of his whereabouts at the time of Madeleine’s disappearance and Mark checks this against the known facts. Mark reaches his series conclusion about whether Christian B is responsible for Madeleine’s abduction and possible murder.
I did think it quite odd why CB would choose to do this now and not before. MWT does have a habit of deviating from the official investigation narrative. I've watched a few of his programmes and the format always seems to be similar. He investigates leads/possibilities that the police have discarded, comes up with a theory, fails to prove anything conclusive, then concludes his theory is still possible.
I can see why CB might favour someone taking this approach to the case in order to cast doubt on his guilt, but fail to see what he really gains from it. It's whatever happens in court that matters and so revealing details of his alibi now, ahead of seeing what evidence the BKA have, does not seem the smartest move. You'd think a person would either state their alibi right from the start in order to protest their innocence and protect their reputation, or they'd say nothing until officially charged.