See:
https://github.com/geoadmin/service-sphinxsearch/commit/cd95dd78f2b84c79d3744d750c7f8e730e96a83c

This would need to be changed to
<pre>
result.attrs.feature_id
</pre>
I could change it on sphinx side to "featureId" though.. But we've been using snake like notation from the very begging when generating indices.