Yes, when sitting the penis is forced to retract to some extent and the fabric needs to yield for allowing this. The spandex sleeve or Sabre Skin will help to minimize the effect. Some fine tuning of the fit whether it be a slightly shorter or possibly tighter sleeve might help also. This might be the adjustment you need to make for maintaining a secure fit while running as well. During the early development phase of the Innerwear, I used to ride extreme mountain bike trails on a near-daily basis, which was the acid test to determine the correct fitting. Careful observation to note the mechanics involved will help you understand what adjustments you need to make.
all the best!