Sounds like the little groove around the piston that it snaps into is just super clean and free of coffee gunk.

I apparently know even less about what the problem is than I thought. After the thorough tear down / clean up / re-lube / re-assemble, it's the same if not worse. Now, with the same daily beans, dose, tamp and slightly coarser grind, I'm getting nothing. Tried twice and could not get any droplets to form on the underside of the basket. Haven't tried a second grinder yet, but that's all I can think to possibly prove the Cremina isn't at fault & that my Mythos is to blame.

I can flush the group just fine and the stream that comes out appears the same as ever.
The lever action and resistance feels the same as ever, and it readily draws itself back down if I'm not holding it up (while flushing, no puck).
Everything about the machine feels completely normal until I try pulling down for a shot. Going to try a second grinder later this evening I guess, but besides that is there anything else that mat be a logical culprit?
I thought this too. After all, it was the first time I've cleaned the piston and inner walls by hand.

What about your filter basket and or portafilter (if it's not a bottomless), are those blocked by any chance?

The coincidence that your grinder is way off seems illogical if you did not touch it, but the again stranger coincidences do happen...

Marcelnl wrote: The coincidence that your grinder is way off seems illogical if you did not touch it, but the again stranger coincidences do happen...

The Cremina is not at fault - it's definitely the grinder. I tried a secondary grinder and was able to pull a sh** with a typical fluffy fine grind setting at a full 18g dose. Now to track down calibration instructions for the Mythis...