My previous tests had used only a simple search (i.e. only the treatment field specified) and they worked. ML provided a specific example of a compound search (year and treatment specified) and I could then reproduce the behaviour he reported. The problem is that I had accidentally deleted this line of code from the code that generates the treatment clause:
if (!empty($where)) {$where .= " AND ";}
I tried treatment=blood-letting and year=1591 and found 2 records, which seems a plausible result as blood-letting alone produces 6 results, 2 of which are in 1591, and 1591 alone produces 90 results, 2 of which are blood-letting.