I don't think you're correct there.
Several years ago, there was no Alderley-Southport. The Alderley services terminated in the main shed at Piccadilly, and it was extremely rare for anything other than a 323 to appear. The Southport services went to Victoria.
May 2018 saw the extension of these terminating services through Castlefield to Wigan, and they became an excellent example of the so-called "random unit generator" effect. It was generally 4 cars, sometimes short-formed to 2 - pretty much any combination of 142, 150 and 156 could be used, and it was very rare to get 4 carriages which were all the same inside and out considering Northern's piecemeal refurbishment programme with different units in different stages of refurb. At some point later they extended through to Southport, but not much changed with the rolling stock.
The random unit generator became a little less random as the refurb programme concluded, and the Pacers were withdrawn - it settled down into usually being either 150+142 or 150+150 (150s of course being mostly refurbished ones by that point). This was until the last of the 142s went, at which point it became almost entirely 150s in a mixture of 2- and 4-car formations (with the occasional 156 appearance). This is how it stayed until the 769s came.
It is expected that things will shortly come full circle, and the Alderley Edge section will go back to being a shuttle to Manchester operated with a 323, with the Southport service terminating elsewhere.