When the query used to retrieve terms to build the value form for a "Taxonomy: Term ID" filter is rewritten, only a single term is returned in the options.
The rewritten query use
SELECT * and when it is rewritten to contains a
LEFT JOIN (for instance, by the Private Taxonomy module), the columns of the additional tables are added to the results. In this case, this cause the
tid of the terms object returned by
db_fetch_object to be null. So at the end, only the last DB result will be present in the build