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I don't think that's original render, when the windows were replaced they probably bashed that edge off and made 'good' after, also why the texture isn't quite the same as the walls. It's usually only on one side of the window as they can lever the old ones out/new ones in.
As said above, bash the loose off and fill with 5:1 sand cement with a pointing trowel trying not to get too much on the cill. At least you've got a 45° bevel edge, I had to do this on all our rear windows and they've got a curved edge. Luckily not to obvious once painted, from a distance with squinty eyes...
I don't have anything helpful to say, but I am interested to know what you eventually find as the solution. I have a very similar situation around the door frame of our side entrance.
I am afraid to tackle it as I worry it will end up being a can of worms when I start to chip away - that the small patch that is visibly damaged now becomes a huge area that falls off.