Its always a good idea to make sure you actually have a document sitting on the scan surface because the scanners can lie to you!
Haha. The preview scan? Yes, that one gets only refreshed when you select a new preview.
I usually have the scan on full a4+ size (without preview) and crop it afterwards to a4 (directly to pdf).
I did change DelphiTwain slightly to also pass back the resolution scanned to set that in the bmp. Somehow that info didn't get into the bmp/timage that I got back.