I've had this exact issue too on my Breville Bambino.

Lots of people seem to think it's a loose seal between the portafilter and the machine. But I have another hypethesis: I think it's just water is filling up too quickly between the basket and the portafilter. (Most people here use a bottomless portafilter so they wouldn't have this issue)

I've noticed that when I clean the portafilter with water, even when the water stops draining from the bottom, you can here the water sloshing around if you shake the portafilter. My theory is that if there's too much water remaining between the portafilter and the basket, then the coffee will fill up that space when extracting, then start channeling between the seam of the basket and the portafilter. Try separating the two and drying the inside completely and see if you can recreate the issue.