I have the same issue. It's because some of the Galaxy A series phones don't have a physical proximity sensor. If you dial *#0*# or *#77692*# you can see "virtual proximity sensor" that doesn't work properly. Maybe they use the lux sensor and touchpad instead of the proximity sensor.
I went to several Samsung service centres to resolve my but there was no solution. As they said this is the design and there is nothing they can do about it. I think Samsung should discontinue or modify this product.
