What you’ve hit is uneven extraction. Parts of the puck were over extracted and other parts under extracted.

Usually the cause is channeling, where some pathways in the coffee puck are easier for water to get through, so they get eroded first which leads to even more water going through these channels. Coffee around these channels then gets over extracted (bitter).

Conversely, much less water is reaching the other parts of the puck, leading to those parts getting under extracted (sour)

Better puck prep helps. Using a WDT tool (some acupuncture needles on a cork would do) or a blind shaker to break up the clumps leads to good results. Making sure the surface of the puck is level after tamping is a big one as well.

What also helps is going coarser in the grind. The coarser the grind, the less puck prep matters and the less channeling occurs. Warning, you’ll no longer be getting the thick crema you may associate with espresso, or the instagram worthy beautiful rat tail extractions. But the coffee produced from coarse grind espresso is IMO much better.

I was taught a lot of this by Lance Hedrick and I applied these learnings to achieve mostly consistent fruity and sweet espresso on most mornings.