I have experienced issues on this several times and even reported it before. I have a filter which is for "NT1683" but I get records which are explicitly entered in NT1684, so there can't be any potential overlap on number of digits entered, for instance. I have this similarly when I am searching for one other 1k square. This leaves me no reliable way to get a hectad list for anything as the column headings perform a simple string search. Maybe I am doing it wrong, but this is quite an important function!