There are multiple issues here being discussed. Some are due to bugs in the game but others are caused by Fanatec hardware.
I use the Clubsport Shifter and have had lots of issues with mis-shifting, mainly between 2nd and 3rd gears, both up and down (in WRC, DR 2.0, AC1, AC EVO). When my shifter was still under warranty it went back and was returned a couple weeks later, everything worked fine for around 3 months then the same issue returned. Both the shifter and handbrake are incredibly sensitive to static/electrical charge. It is a known issue, reported by many people over the course of several years. Handbrakes are the worst culprits, without proper shielding of the wiring and potentiometer, mine would show completely random degrees of engagement between 20-85%. Pulling it to 100% would result in the same variation of recorded input level. Nothing has ever been done by Fanatec to rectify these issues, or even to acknowledge them to be honest.
You can search Reddit to find posts of people taking the PCB (the one with the hall-effect sensor on it) out of the shifter and insulating it with electrical tape before reseating it. There are also people who have shown that the magnet in their shifter is incorrectly aligned in the slot, causing the current gear position to be misread by the sensor. Fixing either or both of these issues has had mixed results, some people report that their shifter has been fine after, others report that the issues returned within a few weeks/months. Either way it will void your warranty if you still have one. These peripherals seem to suffer from bad insulation and don’t hold up well to extended use it seems.