The SVG spec recommends but does not require <defs> appear at the top of the document (before elements that use them).

This SVG, from the MDN patterns page, imports visually to Illustrator (albeit incorrectly):

<svg viewBox="0 0 230 100" xmlns="http://www.w3.org/2000/svg">
<defs>
<pattern id="star" viewBox="0,0,10,10" width="10%" height="10%">
<polygon points="0,0 2,5 0,10 5,8 10,10 8,5 10,0 5,2" />
</pattern>
</defs>
<circle cx="50" cy="50" r="50" fill="url(#star)" />
<circle
cx="180"
cy="50"
r="40"
fill="none"
stroke-width="20"
stroke="url(#star)" />
</svg>


If I reorder the <defs> to the bottom, which is still valid SVG, it does not import anything visual at all:

<svg viewBox="0 0 230 100" xmlns="http://www.w3.org/2000/svg">
<circle cx="50" cy="50" r="50" fill="url(#star)" />
<circle
cx="180"
cy="50"
r="40"
fill="none"
stroke-width="20"
stroke="url(#star)" />
<defs>
<pattern id="star" viewBox="0,0,10,10" width="10%" height="10%">
<polygon points="0,0 2,5 0,10 5,8 10,10 8,5 10,0 5,2" />
</pattern>
</defs>
</svg>