The 'chunk" at the breech could of occurred with real blackpowder also. It also has the ability to absorb moisture from the nipple. The difference between the subs and real black is that real black can re-dry itself right inside the bore without degradation.
Folks have reasons why they prefer real blackpowders.
Other folks have reasons why they prefer a substitute powder.
Retail sales among hunters in this decade indicate a clear preference for substitute powders. I use both & have no personal preference overall.
Not probable given Goex and the othe blacks have graphite on the grains which prevents clumping except under high pressure as when one reloads BPCR stuff with BP. Usually if Goex and its counterparts get wet they flat out die in a grey-whitish goo form... (BTDT)
In the particular instance I cited, I and another gent happened to be both using Goex and had *0* issues under the same conditions and didnt see anything resembling more than a "poof" of powder when those charges were pushed out.
But as you stated, folks are buying more subs. if only because most shops dont want the hassle associated with storage and/or city regs create when selling Goex, Elephant, etc.
Then again iffen one happens to do the reenaction thing or BPCR competition, getting Goex, etc., taint all that big a deal...
Keep yer powerd dry,
D.