At some stage you may have accidently set this. The default is 100%. Besides the obvious slider to set opacity, opacity can be set with ctrl+ scroll and I have previously accidently changed opacity when I meant to do a shift+scroll to change the size of the feathering. However, when I tested on my version of DT and drew a second shape the default returned to 100%, so I am unsure if something else is going on with your system, which I can not replicate. Good luck.