In eZ Find, all filters expect to follow the pattern field:value. If you don't have a colon in the filter, ezfeZPSolrQueryBuilder::getParamFilterQuery() prepends a colon, effectively breaking some filters. This prevents you from adding special filters for the "fq" parameter in Solr, such as frange queries:
An example use case is to enforce a minimum relevance score:
You can get around this by using RawFilterList in ezfind.ini, but that's applied to every query.