Max Verstappen's car has been dealing with an issue with its DRS (Drag Reduction System), but the problem is not of the same nature as that of Mercedes, whose rear wing was declared illegal by the FIA at the Brazil Grand Prix.A fluttering DRS flap was once again seen on the Red Bulls during practice for the Qatar Grand Prix. The issue is especially noticeable when the DRS flap on the RB16B is in the open position.Red Bull F1 team principal Christian Horner claimed the problem was fixed after the first practice session on Friday. There were no problems in the second free practice session, but the issue resurfaced during Saturday's final practice.Sky Sports F1@SkySportsF1More issues for Red Bull and their fluttering wing so far in Qatar 👀#SkyF1 | #F1 | #QatarGP 🇶🇦6:23 AM · Nov 20, 2021558More issues for Red Bull and their fluttering wing so far in Qatar 👀#SkyF1 | #F1 | #QatarGP 🇶🇦 https://t.co/g4wzmOQvUrRear wing problems at Red Bull limited to Max Verstappen's carMax Verstappen is no stranger to faulty DRS flaps. At the 2021 Dutch Grand Prix, the 24-year-old nearly lost pole position to Lewis Hamilton due to his DRS flap not functioning as it should have.Commenting on the issue, Christian Horner told Sky F1:“We saw he lost a lot of straight-line speed just down the last straight and it looks like the DRS hasn’t opened properly.”However, the Red Bull driver hung on to beat title rival Lewis Hamilton to pole at Zandvoort by just 0.038s in a closely-fought qualifying session.Red Bull Racing Honda@redbullracing1️⃣ → 🔟 Pole Positions ⏱ Into Dou-bull figures! 💪 #DutchGP 🇳🇱8:25 AM · Sep 4, 202152074081️⃣ → 🔟 Pole Positions ⏱ Into Dou-bull figures! 💪 #DutchGP 🇳🇱 https://t.co/5GRhwGyAinVerstappen's teammate Sergio Perez, on the other hand, has faced no such reliability issues with the DRS flap on his car, implying that the problem could be due to the difference in set-ups between the two drivers. Red Bull say that while they have improved the condition of the DRS flap, they haven't solved it entirely.Andrew Benson@andrewbensonf1Red Bull statement on their oscillating DRS flap: “We've improved it but we haven't totally solved it, so a bit more work to do on it. It doesn't appear to be an issue on Checo's car. But we will have it under control for qualifying.” bbc.co.uk/sport/live/for…6:25 AM · Nov 20, 20214410Red Bull statement on their oscillating DRS flap: “We've improved it but we haven't totally solved it, so a bit more work to do on it. It doesn't appear to be an issue on Checo's car. But we will have it under control for qualifying.” bbc.co.uk/sport/live/for…Valtteri Bottas was fastest in free practice one, ahead of Lewis Hamilton and Max Verstappen. With Mercedes looking like the team to beat, Red Bull F1 will be hoping to resolve the DRS issue in Verstappen's car before qualifying.