Bordeaux-Begles, nudged into second on Saturday, could not return to the Top 14 summit at Toulouse.
In their first Premiership outing since their automatic relegation was confirmed, Saracens were soundly beaten by London rivals Harlequins.
Top 14 front-runners Lyon won again, while La Rochelle's Arthur Retiere grabbed a hat-trick and Brive left it late.
Rhyno Smith scored twice in the closing stages against Southern Kings to again secure the bragging rights for the Cheetahs.
Exeter Chiefs rallied in the second half against Sale Sharks, but the Premiership leaders still slipped to a first defeat since November.
Although St Helens dominated Super League in 2019, new head coach Kristian Woolf believes the team can still get better this term.
Northampton Saints looked to be heading top of the Premiership until London Irish sprung a shock following Api Ratuniyarawa's dismissal.
Jacques Nienaber had previously served as Rassie Erasmus' assistant with South Africa and has now taken the top job.
Michael Fatialofa is showing "encouraging signs of improvement" and has been moved to the Major Trauma Unit at St Mary's Hospital.
Saracens owner Nigel Wray apologised for his "ill-considered approach" after a report detailed how Saracens fell foul of salary-cap rules.