Can I be certain that there is a stud here?
The window that you depict is set into a rough opening that was then banded on with drywall material. In order for that corner to be finished nice and straight the raw edges of the drywall is usually covered with corner bead strip made of metal or plastic. Then drywall mud fills in the corner from both sides and is sanded to a nice corner finish.
In your case you were probably drilling through one flange of a metal corner beading. Typical flange width from the wall corner may be from 1&quot; to 1.5&quot; (25mm to 40mm). Metal ones are made of thin soft steel material. Here is what a common metal corner beading looks like:

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