the issue is with the proximity sensor, as you now know, was the glass or lcd with glass recently changed? if so either a funnel type of part was left out or the glass is not letting the prox sensor see through properly.
fully tested in quotes. Normally, when I do a screen replacement unless I know there is an issue with the part in the past then I do not test prox sensor. Right now I am seeing this with nexus 5 parts in my area, the glass is not allowing the prox sensor to work properly so with that repair I check it. It would be my guess that the issue is as I said before, new glass is not allowing the prox sensor to 'see' properly. Either that or the prox sensor is just plain broken or unplugged. I don't know the fair phone but i do know this issue.