In July last year, Nick Broomfield released a documentary about the tempestuous life and heart-wrenching death of singing legend Whitney Houston, who was one of the world’s biggest recording stars, but finally succumbed to depression and chronic drug abuse, fatally losing consciousness in a hotel bathtub in 2012, at the age of 48. Horrifyingly, her daughter died in the same way, just three years later.

Now Kevin Macdonald has given us his own documentary study of the same subject. It has been much acclaimed, but I have to confess to finding it a strange and even unsatisfyingly incomplete experience. Undoubtedly, Macdonald got more access to family members than Broomfield ever did and he has much more in the way of archive material, both her on-stage performances and off-stage home videos, apparently shot by her no-account husband Bobby Brown, who is a faintly bad-tempered and unforthcoming interviewee.

The problem is that Macdonald has a huge revelation to unveil around two-thirds of the way in. This is a big story, without a doubt; it would be wrong to recount it in detail here but those squeamish about spoilers or narrative analysis had nonetheless better stop reading. In journalistic terms, Macdonald has done due diligence, although nothing can be proved, and in storytelling terms he can hardly be blamed for holding it back. But, having dropped this bombshell, he does nothing to explore its implications. Really, we need to rewind the film back to the beginning and retell the story in the light of this information. Is he saying that this awful family secret is the key to Whitney’s unhappiness? There is an implication here that it is why she was in fact secretly gay – a pretty questionable suggestion that the film does nothing to justify or challenge.

Fundamentally, the story here is what is already widely known. Whitney Houston was a young woman with a sublime voice, brought up in the New Jersey ghetto, a church choir singer schooled in soul-passion by her singer mother Cissy, and surrounded by music from the moment of her birth. (Her cousins were singers Dionne Warwick and Dee Dee Warwick.) She was beautiful, talented and light-skinned, and Arista Records boss Clive Davis saw how Whitney could be marketed to 80s white America. But Whitney was notoriously barracked at a 1989 Soul Train event for having sold out to mainstream pop, and was also questioned about her close friendship with her beautiful childhood friend Robyn Crawford.

Getting married to bad-boy soul singer Bobby Brown (and turning a blind eye to his philandering) was a way of answering both these attacks. And Whitney also had a taste for drugs stemming from her teen years, and put her shiftless brothers and other family members on the payroll. Some of them were expected to source drugs on tour.

So far, so dysfunctional. Just as with the Broomfield film, there is a strange sense of dancing around the awful truth but never quite getting right to it. Even the big reveal isn’t definitive. The interviewees are clearly holding something back, and, again, just as in Broomfield’s film, no one wants to talk about drugs, at least partly because doing so might expose the speaker to criminal charges.

The key question is: what about Robyn Crawford? Broomfield, albeit in a cautious way, suggested that she was the real love of Whitney’s life and that it was a tragedy that Whitney made a kind of showbusiness career decision to sideline her and favour the obnoxious Bobby, who was to drag her down into co-dependent drug abuse.

This film, however, just skates over the whole issue of Robyn’s existence. Moreover, Houston’s putative status as a gay woman is mostly ignored or – rather insultingly – written off by implication as an abuse symptom. Crawford evidently declined to be interviewed and the legal implications of her non-contribution mean there is a strange and exasperating reticence on this subject. There can be no doubt that she was one of the most important people in Houston’s life, but she is absent here. It’s like a jigsaw puzzle with a giant missing piece.

Well, there are watchable moments, undoubtedly, and it is extraordinary to watch Houston’s sensational performance at the 1991 Super Bowl, singing The Star Spangled Banner with such passion: perhaps the greatest moment of her professional life. Her enigma remains unsolved.