There are 3 infill speeds:
- Infill (the one thats honeycomb by default and of which you can controll the %)
- Solid infill (The area's that would be infill as above, but are to small so there is no honeycomb or anything, 100% infill)
- Top solid infill (the top layer thats visible when finished)
I would try 30 for the first two and 20 for the last one, so you can determine if it has something to do with the speeds. If so, try to find a nice balance.