Enameled aluminium rectangular wire

My company is making enameled aluminum rectangular wires. It is passing all the tests like break down voltage, adherence, flexibility( 4 times thickness and width), elongation. We are having this problem with one customer (A). During winding on a coil former square in shape, the enamel is breaking at the 90 degree bend. Is this curing problem or wire usage problem? With other customers we are supplying same wire without any issues.

We are making bare strip by extrusion and cold rolling process.For extrusion we use water to cool the exuded strip.
With cold rolling we use oil based lubricant and wipe out lubricant before annealing and enameling.
Aluminum is sourced from within India.

What could be the problem.?

My first thought is that if this product is passing your shops tests, and all of your other customers are NOT having a problem, I would suspect that the problem might be with the user's equipment or process. Typically I think you stretch the test sample before you do the bend test. If it passes that test it should not fail when just getting bent 90 degrees. The user needs to take a good look at his machine, the size of the mandrel he is bending the wire around, etc.