That's just a guess, I didn't try to reproduce the issue, but there's definitely something weird going on.
At the very least we should have a look again in 6, but maybe there's an easy fix in 5.
See https://stackoverflow.com/questions/47314562/hibernate-search-facet-not-working-with-lucene-if-index-values-have-nulls

Note: we should check for two cases:

1. There is no document in the index
2. There are documents in the index, but none has its facet field populated