Thanks, this is indeed what I'm referring to. See also:

https://www.thenation.com/article/world/opcw-leaks-syria/

The leaks’ key revelations include:

Senior OPCW officials reedited the Douma investigators’ initial report to produce a version that sharply deviated from the original. Key facts were removed or misrepresented and conclusions were rewritten to support the allegation that a chlorine gas attack had occurred in Douma. Yet the team’s initial report did not conclude that a chemical attack occurred, and left open the possibility that victims were killed in a “non-chemical related” incident.
Four experts from a OPCW and NATO-member state conducted a toxicology review at the OPCW team’s request. They concluded that observed symptoms of the civilians in Douma, particularly the rapid onset of excessive frothing, as well as the concentration of victims filmed in the apartment building so close to fresh air, “were inconsistent with exposure to chlorine, and no other obvious candidate chemical causing the symptoms could be identified.”
Chemical tests of the samples collected in Douma showed that chlorine compounds were, in most cases, detected at what amounted to trace quantities in the parts-per-billion range. Yet this finding was not disclosed publicly. Furthermore, it later emerged that the chemicals themselves did not stand out as unique: According to the author of the initial report, the OPCW’s top expert in chemical weapons chemistry, they could have resulted from contact with household products such as bleach or come from chlorinated water or wood preservatives.
The author of the initial report protested the revisions in an e-mail expressing his “gravest concern.” The altered version “misrepresents the facts,” he wrote, thereby “undermining its credibility.”
Following the e-mail of protest over the manipulation of the team’s findings, the OPCW published a watered-down interim report in July 2018. Around that time, OPCW executives decreed that the probe would be handled by a so-called “core team,” which excluded all of the Douma investigators who had traveled to Syria, except for one paramedic. It was this core team—not the inspectors who had been deployed to Douma and signed off on the original document—that produced the final report of March 2019.
After the e-mail of protest, and just days before the interim report was published on July 6, a US government delegation met with members of the investigation team to try to convince them that the Syrian government had committed a chemical attack with chlorine. According to veteran reporter Jonathan Steele, who interviewed one of the whistle-blowers, the Douma team saw the meeting as “unacceptable pressure and a violation of the OPCW’s declared principles of independence and impartiality.” Interference by state parties is explicitly prohibited under the Chemical Weapons Convention.
The inference drawn from the OPCW’s final report—widely disseminated, including by the Trump administration—was that gas cylinders found in Douma likely came from Syrian military aircraft. An unpublished engineering study reached the opposite conclusion. The study evaluated competing hypotheses: Either the cylinders were dropped from the sky or they were manually placed. There is “a higher probability,” it concluded, “that both cylinders were manually placed…rather than being delivered from aircraft.” At “Location 4,” where a cylinder was found on a bed, the study determined that the cylinder was too large to have penetrated the hole in the roof above; at the other site, “Location 2,” the observed damage to the cylinder and to the roof it allegedly penetrated were incompatible with an aircraft bombing. Ballistics experts also said it was more likely that the crater had been made by an explosion, probably from an artillery round, a rocket, or a mortar. With both cylinders, the study concluded, “the alternative hypothesis”—that the cylinders were manually placed and that the craters were caused by other means—”produced the only plausible explanation for observations at the scene.”