This bug is the same issue as
LUCENE-7231, just for MultiPhraseQuery instead of PhraseQuery. The fix is the same as well. To reproduce, change the test that was committed for LUCENE-7231 to use a MultiPhraseQuery. It results in the same error
java.lang.IllegalArgumentException: Less than 2 subSpans.size():1
I have a patch including a test against branch_5.x, it just needs to go through legal before I can post it.