I've had lots of issues with brake squeal, and still do.
When I bought the car, both fronts were squealing. Rears have always been fine. Got both sides front pads changed, right side squealing gone but left was very loud at moderate brake pressure, heavy pressure and very light pressure you hear nothing. There is a sweet spot for it, but I'm always using this sweet spot of pressure!
Couple weeks later I got a rear wheel bearing done by same mechanic and asked him to sort the front squeals out too (he said they need cleaning and chamfering), he did sort it out. Squealing definitely went, which suggests that the cleaning worked. There is copper grease all over the pads too. But after about a month or two, it gradually came back, and now its quite bad. But it seems to notoriously be mainly the left side. Mechanic said the pads were seized onto the calipers too, although don't know if this has any relevance.
It's annoying and would like to sort this out sometime.