Support on build plate only doesn't create supports only on build plate
I'm fairly new to PrusaSlicer, I'm coming from Cura.
Using mostly stock settings on Ender3, only I'm using 0.5mm nozzle. Slicer version is 2.3.0.
The part itself is Cylinder_Head from https://www.thingiverse.com/thing:644933
Not sure from this movie where you think it creates supports not originating on the build plate?!
This is a known issue with all the builds of PS so far. PS is VERY aggressive at expanding support outwards for anything its supporting. This means it will grow out onto the part too as long as it STARTS on the build plate. There are numerous github issues relating to this.
If you are using stock settings just make sure your contact z distance is set to a minimum of 0.2. I'm not familiar with the ender defaults but the prusa ones usually have this set to 0.1 which makes them weld together.