Briefly speaking, ‘Platte’ of UB20.2 disturbs that the diffuser works.
Please see explanations below.
1. Pressure Comparison
I found some features in the pressure result of your simulation UB20.2 comparing with UB20.
- Low pressure on the upper side surface of the diffuser
- High pressure on the upper side surface of the floor in front of ‘Platte’
- Higher pressure on the lower side surfaces of the floor and the diffuser
It seems that the area in front of ‘Platte’ increases some downforce, but the area is not large.
On the other side, the low pressure area of the upper side of the diffuser and the higher pressure area of the lower side of the floor and the diffuser increase lift force. And the areas increasing lift force cover larger.
I think that lift force is generated as the sum of those pressures.
The difference of the pressure between the upper and lower surface of the diffuser UB20.2 also generates negative drag.
2. Flow Comparison
‘Platte’ makes a vortex flow behind it.
The reverse flow the vortex affects on the diffuser as greatly disturbing the diffuser works.
These are explanations lacking in engineering rigor, but I hope these help you.
All the best,