A 2014 History article says
According to new research by a Canadian historian, the 1918 flu outbreak that killed 50 million people originated in China. [...]
Humphries found medical records indicating that more than 3,000 of the 25,000 Chinese Labor Corps workers transported across Canada beginning in 1917 ended up in medical quarantine, many with flu-like symptoms. The influenza ripped through the Canadian guards and soon took root in North America. [...]
The mystery of the origins of the “Spanish flu,” is not totally solved, however. “Only DNA testing of samples from these earlier outbreaks can truly confirm or deny the theory,” Humphries acknowledges.
His evidence/argument seems based entirely on the movement of people and occurence of these outbreaks of some kind of flu among Chinese Labor Corps. Has there been any biological research on these earlier outbreaks, proving or disproving a link to the later "Spanish flu"?
I see there's a (more recent) 2016 paper by GD Shanks which disses the idea, to the extent that it might have been the source of the Spanish flu in Europe
Laborers and soldiers from China and Southeast Asia recruited during the First World War by Britain and France have been suggested as the origin of the 1918 influenza pandemic in Western Europe. This study aimed to review the available data to better understand the sources and origins of the 1918 influenza pandemic, and clarify whether, in fact, there was an Asian connection to its onset. We reviewed official mortality lists from the Commonwealth War Graves Commission and the French Ministry of Defence for all-cause (Britain) and pneumonia/influenza (France) mortality, respectively. The results indicated that influenza mortality (estimated 1/1000) in Chinese and Southeast Asian laborers and soldiers lagged other co-located military units by several weeks. This finding does not support a Southeast Asian importation of lethal influenza to Europe in 1918.
I also see that a 2019 review by Worobey, Cox and Gill is more directly dismissive of the idea that Chinese laborers could have transported it through Canada, but again on a historical argument:
Our view is that attempts to pin down the exact location of the first cases of any influenza pandemic are fraught with difficulties, and, for at least two of the three hypotheses noted above there exists strong contrary evidence. The simplest objection is to the idea that Chinese labourers were responsible for the spread of the pandemic virus. Crucially, Shanks  showed that influenza cases among Chinese and Southeast Asian labourers and military recruits lagged, rather than led, cases among other groups in the same locales. Moreover, Chinese labourers were also shipped to Europe from the East via Suez or the Cape, although these routes were rapidly abandoned in March of 1917 in favour of transporting them across Canada . Finally, although the Chinese workers were reportedly transported across Canada in sealed trains, it seems unlikely even under this harsh scenario that the virus would not have initiated detectable spread in Canada if these individuals really were the original hosts of the pathogen.
In their 2019 review, Worobey, Cox and Gill also point to the 2014 genetic analysis of Worobey, Han and Rambaut as the "state of the art" in terms of genetic knowledge on the Spanish flu, in which they argue it was more probable of "Western hemispheric" origin.
Phylogenetic analyses have also been used to model 1918 virus origin but have yielded different dates for the estimated origin of the pandemic virus (36, 76, 79, 80). Until pre-1918 influenza A virus sequences become available, reconciling the dates of the origin of the 1918 virus phylogenetically will remain difficult. It seems highly unlikely on epidemiological and biological grounds that a virus expressing the pandemic H1 HA, with its inherent virulence, could have widely circulated in humans much before 1918.
(In there, ref 80 is Worobey et al. 2014.)
I guess that means there's no other/biological research on this putative China link presently, but I'm leaving this as an "open question".