Chanelling over puck edge

Hello community! This is how my puck looks after almost every shot done. 50 cent has less holes in his body. Sometimes the whole perimeter is covered with holes, sometimes only 3/4 perimeter. But every times it’s chanelled.

Setup is: NS Aurelia 2 T3 with IMS shower screens, 17g LM strada baskets, Mythos one, Barista Hustle tamper with new base, ocd knock off. Grinder burrs and chamber are cleaned 1-2 times a week (consumption is Little, up to 2 killo/day), groups are backflushed frequently, shower screens are washed every 3-6 hours, at the end of the day with detergent.

Situation remains the same if I change pump pressure (switched to 6 from 9 bar, flow rate is 150g/30sec), temperature (92,4-93,6°c), distribution technique (palm tapping or tapping plus tool), dose (17-18,5), baskets (to 20g ims sometimes), grinder (ek43t sometimes).

Shot usually looks normal, first drop at ~6 second, normal colour. Body is light, watery perhapse. Tastes bitter in after taste. There is a rather darker halo on the bottom of the puck that matches holes area.

Any ideas what the problem can be?

P.s. Sorry for heavy sentence constructions, I have little practice in English.