From the pull request mb-solr#60 by bitmap addressing SEARCH-732:

We would otherwise need to copy the changes from [1] into our MusicbrainzTokenizerImpl.jflex file - though regnerating it with 1.9.1 produces many other changes in the tokenizer which I don't know the implications of. At the very least we'd have to rebuild all indexes.

JFlex was updated to 1.9.1 in 692c9ab (I assume only "for good measure").

[1] apache/lucene@af831d28

MusicBrainz Solr cannot stick with an old version of JFlex for ever. This upgrade has to be made ready as soon as possible - before it becomes a blocking point - and later on deployed along with other changes requiring to rebuild Solr collections (search indexes).