Solr 3.4 group.truncate does not work with facet queries

When using Grouping with group.truncate=true, The following simple facet query:
facet.query=Monitor_id:[380000 TO 400000]

Doesn't give the same number as the nGroups result for the equivalent filter query:
fq=Monitor_id:[380000 TO 400000]

From the Wiki page: 'group.truncate: If true, facet counts are based on the most relevant document of each group matching the query.'

If I turn off group.truncate then the counts are the same, as I'd expect - but unfortunately I'm only interested in the grouped results.

Asked this question on the Solr-user mailing list and was told it is likely a bug by: Martijn Groningen.

I'd be very interested in any workaround for this bug!