Tim-
Jim, Abe and Dan can speak to the sensory stuff WAY better than me, as can many others here I am sure. My guess is the first of your shot was a bit bitter from fines and/or high temperature at the start of the shot.
A fun thing to do, and to help you explore the shot's extraction a bit more, is to split a shot into thirds. I would actually recommend that you split into 4 parts and overextract the shot. Line up 4 shot glasses or cups (warmed preferably), shoot for a proper shot in 25-30 seconds, pull the first 8-10 seconds or first third in the first cup, next third in the second cup, final third in the third cup, and for kicks another 10 seconds or so in the fourth cup.
Taste each individually and see what you think of each.... take notes, and report back. I'm not gonna tell you what you'll find out as to not influence your opinion

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