Spouted portafilters are made to split the flow in 2 whatever is happening in the basket, even with the poorest distribution. The fact that coffee is consistently coming out of the same spout first is actually a sign that it does not come from a poor distribution, otherwise it would sometimes come out of the other spout first.
IMO, you machine is simply not leveled.
Also, you can convince yourself all is going perfectly well inside your spouted portafilter, but the only way to tell is to use a bottomless one. I too was convinced all was great before I started using a bottomless :-)
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