Scriptwriters love to feast on the lives of children’s authors. The themes tend not to vary: they may have brought happiness to millions of children but their stories — sob — were fertilised by unhappiness. Saving Mr Banks: Mary Poppins author was a bossy shrew because her alcoholic father died young. Miss Potter: Peter Rabbit creator never found love. Finding Neverland: Peter Pan playwright cheered up grieving family. Enid (made for BBC Four): Miss Blyton was a monster traumatised by her upbringing.

And so it will presumably go on. We can probably not expect a family film about Charles Dodgson taking cute snaps of little Alice Liddell, but one day, years from now, skint single mother Jo Rowling, played by an as yet unborn actress, will chew once again on a Biro in an Edinburgh café and conjure magic. For now, there’s Goodbye Christopher Robin.

It’s the same sort of story. In this iteration, A.A. Milne was a cold father defrosted by his young son’s imagination, but whose tales and poems would eventually make the boy’s life a misery. Dark shadows presage naught but ill from the start. In 1941 a long-faced postwoman delivers a brown envelope to the Milnes’ leafy cottage in Sussex, which can mean only one thing. We duly flash back to the Somme, the bad show which convinces shellshocked playwright Alan Milne that no good can come of conflict.

‘Blue’ by nickname, blue by nature, Milne flinches when corks or balloons go pop. Then his wife Daphne (whose actual name was Dorothy) gives birth to little Christopher Robin, whom they call Billy Moon because he can’t pronounce his surname. They’re not the best of parents, but back then who was? Milne carries the infant as if he’s a tea tray, and Daphne styles the boy in girly smocks and a Louise Brooks bob before handing him over to Nanny (aka Noo) for years and years.

There’s a sweet sequence that captures the transition from child’s play to literary invention. Huffy Daph dashes up to town to become a flapper and punish Blue for his writer’s block. Then Noo disappears to attend her mother’s death. And so the father is left in charge of a cross-dressing son with a menagerie of stuffed animals and a fondness for the woods.

E.H. Shepard motors down with his sketchbook. Soon Winnie-the-Pooh is flying off the shelves and the bear’s innocent young owner is a celebrity whose image is exploited to shift ever more units. His father rings from America to bid him goodnight and the call is broadcast live on the radio. He does a Q&A for a phalanx of stalker fans. He’s trolled by the Times, and required to pose right next to London zoo’s large Canadian bear. Isn’t it funny how a bear makes money?

It’s an affecting story, competently visualised by director Simon Curtis, who made the pleasant period bauble My Week with Marilyn. And yet there’s a gnawing credibility gap; the facts feel mangled implausibly out of shape to serve the narrative’s needs. The exploitation of Billy takes place in a vacuum, with no one seeming to sanction it. Daphne sends a poem by Milne to Vanity Fair, which publishes it without telling him. Nanny is ruthlessly sacked for having a beau. Christopher’s brush with death in wartime feels like a manipulative fantasy. Meanwhile inconvenient facts have been deselected. The blurb before the end credits advises that Milne’s pacifist tract Peace with Honour was published in 1934. It omits to mention that its counterblast, War with Honour, followed in 1940. And that the older Christopher Robin, who eventually makes his peace with his fictional alter ego, drifted apart from his parents and spoke to his mother once in the last 15 years of her life.

There’s also an issue with accent continuity. On vowel-contorting duty as the spiffingly English Milnes are an Irishman (Domhnall Gleeson, ever adept at playing glassy-eyed oddballs) and an Australian (Margot Robbie). Neither ages a scrap in 20 years. Will Tilston as the scrumptiously dimpled Christopher Robin looks jolly 1920s but sounds massively 2010s. Kelly Macdonald makes a fine Noo.

It’s not clear who Goodbye Christopher Robin is for. Children may emerge with PTSD from a story of neglect bordering on abuse. Perhaps its natural market is across the pond, where Pooh’s image rights are owned by Disney and the original stuffed toys are in the New York Public Library. Might Americans see this tale of crippling emotional continence as an adorable specimen of heritage Britishness? Do bears shit in the woods?