Hello experts

I've been lurking here for a while. Thank you all for sharing and allowing us all to learn.
I have GS3mp and Weber KEY. Recently I've been discovering the world of lighter roasts and acidic flavors and learn to like it.
While working with these light roasts I'm experiencing some weird behavior that I still not able to solve.

I tried to catch this behavior here: I'm using 17g LM basket with 18g of coffee. When I start extracting everything looks good. I have some 2 bar pre infusion (changes between 10-30 seconds), and then raise the pressure to 6 bars, and then 7-8. Sometimes straight to 7-8. The extraction looks good at first, but then starts to change and gets this weird channeling where the extraction chokes and separates to different paths that still unite to a single flow:

The rate itself doesn't change. At the end usually the puck is stuck to the group head.
The taste is good. I'm able to play with grind size and pressure to control the flow for different profiles and it affects the flavors, but I always get this weird behavior.
I have it with different beans, not specific type. But it doesn't happen with darker roasts.

Any help/tips will be appreciated.

I don't think this is channeling, light roasts are just thinner bodied and you're not going to get the same viscous cover. It's flowing through the puck all over the place. These flows just collect into streams on the basket due to surface tension. Especially toward the end of the shot as it thins out.

It happens with darker roasts too but they are thick and foamy which covers everything up. They flow like this too but when the output is thick you don't see it, just like you don't see it here until the end of the shot as things thin out.

There are probably some things you can do to improve the look or evenness slightly but if the shots are turning out well you're probably fine.

As far as stuck puck on the group you may just need to dose slightly lower if it is compressing against the group too much.

By no way am I an expert but it does look a little bit like when I grind too finely. Perhaps there are some clumps in there that create the blind spots. Lighter roasts can be a bit finicky to prep, I suppose because of the need to grind finer. Grinding a bit coarser and hitting the pack sooner with higher pressure might help. Maybe.

Another option could be the fines migrating down the puck and clogging the basket. You could try and put a filter paper on the bottom of the basket to see if it still happens. Caveat is you may have to grinder finer still.

My pull looks very similar as yours (behavior wise).

In my experience light roasts don't pull like dark classic roasts. It depends on the light roast, some roasters do omniroasts which finish first crack light for drip but go longer on the roast for a maillard reaction that allows more extraction out of a shorter high pressure espresso pull. In theory I think the density of a light drip roast would be tough to espresso extract enough sweetness from in a way the acids wouldn't dominate. That wouldn't be a problem with the high temperature, long extraction of a pourover drip.

I don't get thick syrupy crema from a light roast even if the grind is fine enough for a ristretto. Compared to a classic roast pull I shoot for lower dose, higher brew temperature, a bit longer preinfusion, higher extraction pressure start and cut the end off the shot a bit. I also go by taste and how the spent puck looks rather than how the extraction looked.

Yeah the lighter roasts toward the end just tend to collect into localized streams before coming together. It's kind of like multiple pony tails, you can see the scalp between the bundles where the hair is parted but that doesn't mean these are bald spots.

Thank you for your replies. Good to hear this is normal behavior. I do like the flavors I get when I grind as fine as I do, so I'll continue

I'm about to get my Unibasket and it will probably come with whole different behavior once again.

After you completed the preinfusion and the coffee began dripping into the cup, how long did your extraction last? I couldn't tell exactly from the video. Even though the coffee tasted fine, if the extraction took more than 35-40sec after the first drips, it could be that the coffee was ground finer than need be. I don't have the same machine as you do, but I've seen similar appearing extractions from light roasted coffee ground a bit too fine. It tasted OK, but was a bit overextracted and tasted even better when ground less finely. Depending on the elevation where the beans grew, lighter roasts in my experience extract better with coarser grinds than darker roasts.