I’ve had this Ender 3 V2 for about 6 months, and it has been printing great, but every now and then I will get a brittle messed up top on my prints. I can simply crush these tops by just touching them, they are super weak. The odd thing is it does not happen on every print.
Most likely this is a symptom of an under extrusion issue. Could be heat-creep. Could be caused by a overly aggressive auto cooling setting where the minimum layer time is set so low that heat has time to climb up the filament and heat break causing a jam. Or other related reason such as the print speed is very low at a specific point in the print. This type jam can self recover with a temperature cycle of the nozzle so might not be noticed as the next print could be started without any intervention to clear it. The weak area of plastic could simplify be what oozed out of the nozzle after the jam.
Given the limited information I cant say much as to the cause.