ixidized pads ?

We have received in one lot PCBs with defect of not wetting properly. Pads were finished with HASL but they were matte rather then shiny (attachment). For me looks like they are oxidized. Interesting is there was just top side with such defect. We have treated them with izpropanol and it worked.

What could couse such defect ? What exactly our PCBs manufacturer did wrong during process ? I think those PCBs were exposed for a long time on high temperature and it coused oxidation.

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