Porting bug: Parsoid-PHP probably assumes that the "i" (template index in data-parsoid) property is always set in the parts array of data-mw. But, VE doesn't insert this.
Example output:
<span typeof="mw:Transclusion" data-mw='{"parts":[{"template":{"target":{"href":"Template:Quote","wt":"Quote"},"params":{"text":{"wt":"something"},"sign":{"wt":"some"},"source":{"wt":""}}}}]}'></span>