Unsure, but different styles do depend on different water chemistry. Dark malts make the mash more acidic, which can counterbalance water with a naturally high pH. Naturally high pH doesnt' work as well for pale ales because the mash isn't acidic enough. Where abouts are you as places like the south of England have high Calcium levels and pH.
Or the add flavour could be a variety of things. Chlorine/chloramine can react with the phenols in the beer to produce chlorophenols (TCP flavours).
Whas is the 'strange' flavour that you mention. If you can try to describe it as best you can, people on here will likely be able to suggest possible causes/solutions.
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My water is pretty neutral. I've done brews with both untreated and treated with lactic acid and also use campden tablets for the chloramine. I've used dry yeast and liquid yeasts and don't think they were underpitched as I only do 11L batches.
It's hard to describe the taste. It's like a really sharp bite. My first thought was astringency but I've both squeezed and not squeezed the grains and had the same result