There is a possibility it is a number of factors creating the loss of
adhesion. 
1) The copper at that point may have some residue which is invisible to the
eye causing the lift off.
2) If at an extreme turn in the trace, you may have some air trapped
alongside the trace which is trying to come out when reheated. 
3) The Tg of the mask is too low. ( this would mean that the more than the
small traces would come off regardless of size.
4) The mask is too thin and that particular set of boards have been over
cured.
5) What usually causes lift off in only particular areas is a combination of
1 and 2. The flux can penetrate micropores and when heated expand to 22.4
times its volume which is more than enough to lift mask off the board. When
you clean it will take it right off. 

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