As described in a dev@ list thread, the value of FacetResult#value should reflect the number of docs containing at least one value in a given facet path. LongValueFacetCounts counts the number of values contributed by all docs. In cases where all docs contain a single value, this is fine, but if a doc contains multiple values, FacetResult#value will be incorrect.
This is a simple fix so I think we can include it in 8.9.
Note: Spinning this off from LUCENE-9952 since fixing this for all cases (particularly SSDV) is trickier and may require non-backwards compatible changes.