this is most likely a masking fill issue. DWG will not include fills with a foreground/background pen. There are a few things you can do. Firstly, set a graphic override or elevation marker setting to turn all fills white and avoid 100% solid fills or any screen fills. At the very least, you should have a graphic override rule for all fill background pens to be transparent (pen 0). Next, you can adjust the dwg translator to NOT save background/masking fills fills.
BIM solutions and trouble shooting (self proclaimed) expert. Using Archicad 26 5002 US on Mac OS 11.5.2
Thank you very much for the reports and I am very sorry for the issue!
I made a quick elevation in Archicad 25 INT 5005 and export it to DWF (with color and shadow turned on), however I was unable to reproduce the issue. Could it be possible that you have enabled Texture fill in the Elevation settings?
If not, I suggest reaching out to the local support with the project plan and steps to reproduce, so that they can start the investigation from here.