TestMoreLikeThis.testMultiFieldShouldReturnPerFieldBooleanQuery assertion error [LUCENE-7161]

I just hit this unrelated but reproducible on master #cc75be53f9b3b86ec59cb93896c4fd5a9a5926b2 while tweaking earth's radius:

```
[junit4] Suite: org.apache.lucene.queries.mlt.TestMoreLikeThis
[junit4] 2> NOTE: reproduce with: ant test -Dtestcase=TestMoreLikeThis -Dtests.method=testMultiFieldShouldReturnPerFieldBooleanQuery -Dtests.seed=794526110651C8E6 -Dtests.locale=es-HN -Dtests.timezone=Brazil/West -Dtests.asserts=true -Dtests.file.encoding=US-ASCII
[junit4] FAILURE 0.25s | TestMoreLikeThis.testMultiFieldShouldReturnPerFieldBooleanQuery <<<
[junit4] > Throwable #1: java.lang.AssertionError
[junit4] > at __randomizedtesting.SeedInfo.seed([794526110651C8E6:1DF67ED7BBBF4E1D]:0)
[junit4] > at org.apache.lucene.queries.mlt.TestMoreLikeThis.testMultiFieldShouldReturnPerFieldBooleanQuery(TestMoreLikeThis.java:320)
[junit4] > at java.lang.Thread.run(Thread.java:745)
[junit4] 2> NOTE: test params are: codec=CheapBastard, sim=ClassicSimilarity, locale=es-HN, timezone=Brazil/West
[junit4] 2> NOTE: Linux 3.13.0-71-generic amd64/Oracle Corporation 1.8.0_60 (64-bit)/cpus=8,threads=1,free=409748864,total=504889344
[junit4] 2> NOTE: All tests run in this JVM: [TestMoreLikeThis]
[junit4] Completed [1/1 (1!)] in 0.45s, 1 test, 1 failure <<< FAILURES!
[junit4]
[junit4]
[junit4] Tests with failures [seed: 794526110651C8E6]:
[junit4] - org.apache.lucene.queries.mlt.TestMoreLikeThis.testMultiFieldShouldReturnPerFieldBooleanQuery
```

Likely related to #8011?

---
Migrated from [LUCENE-7161](https://issues.apache.org/jira/browse/LUCENE-7161) by Michael McCandless (@mikemccand), updated May 19 2022

Run the provided ant test command for TestMoreLikeThis.testMultiFieldShouldReturnPerFieldBooleanQuery and inspect TestMoreLikeThis.java around line 320. Compare the failing assertion with the multi-field query behavior and related issue #8011. Done means the reproducible assertion failure is understood and the targeted test passes reliably.

Written by the indexing model from the issue text.