PrusaSlicer 2.3.0 output underextrudes first layer
I've just got my i3 MK3S back up and running after several months out of use. I've upgraded to the MK3S+ extruder and added a new Phaeton Dragonfly hotend. Everything went together fine and the bed levelling and 1st layer calibration was great. Ran off a 0.2mm 75x75mm test square and the result was a lovely, flat, well-bonded square. Did my first actual print (a M10 Bowden filament sensor cover so nothing complex) and the first layer just looked awful and I could visibly see that there was not enough filament being extruded. So I naturally thought that it was under-extruding due to build error or that I'd got a setting wrong in PrusaSlicer 2.3.0 so I went back to the test prints, which worked perfectly. Checked the filament pathway, Bondtech gears etc. and all was fine; checked Slicer settings and couldn't see anything obvious but then I hadn't changed many settings from default.
Then I opened up the print file in a Gcode viewer and can see that the Gcode produced by PrusaSlicer was running an volumetric flow rate of about 0.98 mm3/s on the first layer. When I opened up the test square Gcode it was running at about 3.4 mm3/s. Downloaded Ultimaker Cura and set it up with the generic profiles for i3 MK3S and PETG, changed none of the settings other than the nozzle temp, and it prints perfectly.
Does anyone have any idea why Slicer would be generating a first layer with such a low extrusion rate? What setting could be causing this and any ideas how I can change it to prevent this? I would expect PrusaSlicer to work perfectly with an i3 MK3S with default settings and Cura to need more adjustment but it seems the opposite is true.