The issue being encountered is a likely due to an under-cut of the
soldermask during the white tin application process.  White tin requires a
microecth of the copper surface prior to exchange of the tin to the board
surface.  This microetch, and certainly a re-processing of boards typcial
to white tin finishing, begins to affect the bond of the soldermask to the
copper along the permeter of the land areas.  The smaller the perimeter
area, like a trace into the fine pitch device, has a propensity for
increased etching from the energy created from the chemistry.

This condition will be typical in a IT, IS and ENIG finishes.  Genrally IT
and IS can be re-processed for issues from storage and this amplifies the
condition.  Be sure that your pre-heat and reflow temperatures are not
increased to high for the lead-free soldering - IT should process similar
to solder if shelf-life, storage and handling practices are controlled well.

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