Very fresh beans will do that as well. They start good then go gusher. The crema will go flat very fast. If you are channeling, you will typically get a bitter acrid taste in the cup. I had a Millennium for quite some time, 18 grams is quite a bit in the stock double basket. You could try letting your beans rest another day or two or cut your dose down a gram or two, tighten the grind a little and see what happens. Without a bottomless portafilter it is hard to say. You will eventually learn what a channeled shot tastes like, along with a young bean, old bean and under extracted and over. Channeling is often similar to a over and under extracted cup. Light body with a bitter finish since the area channeling is over extracting and the rest of the dose is under extracting.

Every blend/machine/grinder behaves differently. You just have to try it out and see what happens. When I had my Mazzer/Isomac combo I think I use to run in the 15-16 gram range but the dose can swing a lot from blend to blend. If you prep you portafilter, lock it into the group and then remove it, is the puck scuffed, cracked or otherwise disturbed? If it is, you could be disturbing the coffee bed and creating vertical or horizontal channeling.

If the grinder is new to you, you may be getting a new dose based on volume because of how the grinder grinds. Are you weighing the basket after dosing on a 0.1 gram scale? That would verify/eliminate that possibility.

There is nothing wrong with an 18 gram double, but if you are having problems, changing dose is one variable you can play with. In the end, the only thing that really matters is what you like. If the taste improves at a lower dose or extraction volume, compensate by using a little less milk in the drink to keep the milk to espresso ratio close to what you like.

The answer is really not that complicated .... every shot you've ever pulled has channeled, whether it seems that way or not. Your shots will continue to do so until a) you take an expensive espresso course, or b) you get a naked PF and learn how to distribute and pack.

Some of us older regulars had been making thousands of espresso shots per year for many years (over twenty for me). And when we got our first naked PFs, we still got squirted in the face for a week or so.

Still happens now and then.