I asked @NW9 to post here as I thought it would be helpful to hear everyone’s opinion.
Personally I often see similar bottomless shots myself with medium and light roasts (Minima and Niche combo).
I don’t use a leveller anymore, I attach a magnetic dosing ring to the basket, dose coffee into it from the dosing cup, WDT with 0.4mm needles (0.3mm might be better), tap on mat once and ensure grinds are level before using a self levelling tamper.
Often there is no sign of channeling yet significant dead spots occur. @Rob1 has previously suggested that dead spots may occur due to fines blocking the basket holes. With medium/darker roasts these dead spots may still occur although hidden due to more crema. Or maybe there are fewer dead spots as medium/darker roasts usually require a coarser grind.
It’s possible the mazzer burrs produce more fines when the grinder is new and with use/seasoning this will improve?
I would be curious to used a different and/or seasoned grinder to see if the bottomless shots differ with the same espresso machine and beans.
I agree a slightly finer grind may be beneficial though.
@Rob1 what are your thoughts?