I have the same problem when 0.2mm stainless steel nozzle was used. The first layer was very thin and rough especially with smooth plate. Anyway, I figured a workaround that can be done with a couple Bambu Studio setting.
For the thin layer, you can compensate that with Quality -> initial layer height. Just add whatever the actual thickness that is off from the desired. For example, if the layer height is 0.1mm and the actual first layer thickness is 0.07mm, then make the initial first layer to 0.13mm. This will restore the thickness back to 0.1mm.
Once changed, test print first layer again. If the layer is still rough, then adjust Quality -> initial layer flow ratio. The starting point is relative to the initial layer height compensation. For example, if you need to compensate initial layer height by 0.03mm to get 0.1mm layer, then it is 0.1 / 0.13 = ~0.77, which is the initial layer flow ratio. Why? printer extrusion of first layer is for 0.13mm thickness, but we will only get 0.1mm. This means printer should only extrude 77% of filament for 0.13mm layer in order to get 0.1mm layer.
To Bambu Lap … please fix the issue in the next Bambu Studio release. It should be a simple fix. Likely just some gcode update.