Method mustRewriteQuery(SpanQuery) returns true for SpanPositionCheckQuery, SpanContainingQuery, SpanWithinQuery, and SpanBoostQuery, however, these queries do not require rewriting. One effect of this is e.g. that UnifiedHighlighter does not work with OffsetSource Postings and switches to Analysis which of course has consequences for performance.
I attach a patch for lucene version 7.6.0. I have not checked whether it breaks existing unit tests.