Why does an I inside a span inside a button show up as IJ in Safari

I wrote the following code

<button>
<span style="font-weight:500;">I</span>
</button>

but when I run it on Safari I get a button with IJ instead of I written on it. It works just fine on Chrome.

Safari

Chrome

I've noticed this only happens when I have a span inside a button and the font-weight is exactly 500. This issue happens whenever there's an I, for example, if the button read TRY IT NOW it would show up in my Safari as TRY IJT NOW.

I'm running Safari 9.1 (10601.5.8.3).