Sorry did not see the svg first only the screen shot. As Monica implies it is teh rules of mathematics that cause your problems. Offset path is calculated perpendicular to the stroke and as you are coming to a tangent the miter would need to be infinity (because parallel lines never meet) if you round the point or make it so that it is not infinite you will get a correct path. Offset with round will work but never with miter at this angle, you must round the angle or make the handles meet as a v-
Are you sure a miter is what you want? I don't think you want an unmodified offset path because it would at best look something like this:
I tried to upload an ai file but seems it does not work, why I wanted to give you an ai is that Illustrator allows to use your original as a guide.
I couldn't stop without showing how I would solve such a situation.
First mae a copy of your shape and made it to a guide, so I could make sure I was true to your original.
I deledet the side points on the top part of the cross and modified the curves to simplify it even further.
Next I offset the path getting a similar result to the image you first got.
I removed the top segment.
Aligned the top points of the ofset path to centre of selection and joined them.
I moved the top point of the offset path uppward and then adjusted the path so that it had the same flow as the inner shape.

It is not quite parallel and not a true offset path but I imagine this is what you were aiming for.
Hope your project works out 🙂
Here is an svg of my result (did not manage to get ai file to work correct)