It is quite possible the slit is supposed to be there. I checked other brand new Dorman diaphragms and the ones on my '12 Traverse, '10 Sierra, and the '14 Sierra 1500 at work. The slit was present on all. There was even a slit on the brake reservoir of the Kubota mules at work.
Picture of the slit in the Dorman brake fluid/clutch reservoir:
The only reason I can rationalize this diaphragm having this as designed is it may relieve pressure from heat. I have mentioned on my build post that back in May I noticed fluid on the atmosphere side of the diaphragm. You would think that brake fluid cannot find its way here if the diaphragm is sealing correctly, but I believe it is coming thru the slit, and finding its way onto either the brake reservoir and or the clutch reservoirs like the GTO reservoir or my adaptation of a the GM diaphragm to the Wilwood cap.
If many remember there have been posts concerning the possibility of:
-the GM brake reservoirs leaking at the seams
-the cap diaphragm not sealing do to some thinking the cap does create enough force to seal the diaphragm. Some have added a thin spacer at the cap to squeeze the diaphragm tighter.