I have seen some very strange things with leaded windows, some of it well before I started with wfp.
Lead bubbling up over the joints, (which if brush bristles get under will lift it right off in strips)
Lead that has moved across the glass even though still stuck firmly to the glass, (have seen this numerous times both before switching to wfp and after, both outside and in between double glazing) and before they had ever been touched by a professional window cleaner.
Lead peeling off the glass between the double glazing (obviously nothing to do with any cleaning method) and some that has just popped off the glass with no apparent warning when my brush has gone over it (poor adhesion as if fitted correctly with quality materials this wouldn’t happen).
I have a number of customer’s whose windows are very old, 20-30 years, and the lead on them is still firmly in place even though the seals have gone on some and there is condensation visible between the panes.