higher longitudinal slip will lead to lower lateral friction, this is an inherent behavior of the pacejka tire model. the combination of lateral and longitudinal forces can be configured via the lateral & longitudinal combined slip parameters of the tire model (for tire96 and mf6.1). fgear has accurate implementations of these tire models but tuning tire models is a black art.
tire model is one side of the story though. another important factor is the drivetrain simulation. fgear’s drivetrain is not simulation grade, has some approximations and possible inaccurate parts. this may or may not contribute to the problem. rewriting the drivetrain could only be a subject of a future fgear2.0 project which is currently not planned.
for all those asking for drifting physics I usually say fake it until you make it but that requires the necessary knowledge to modify things around the code and a lot of time & experimentation.