wow .. you are well beyond most of the people that frequent these boards. matched pairs should not be needed since you are looking at a good opamp though BJT singles will not work since you are looking at low currents and will always be fighting the beta tempco.
in the case of BJTs you have the base collector leakage multiplied by the very temperature sensitive beta. there are circuits that can compensate for that and you are obviously using one. I am assuming you are using a sense resistor, though you do not mention how much sense voltage you are using.
how they measure it ....
the
https://www.electro-tech-online.com/custompdfs/2010/07/BC846ALT1-DPDF-1.pdf specs it at 15nA@30V Vbc but expands it to 5uA@150C ... before you take that at face value you need to look at the actual symbol: Icbo
that means the base is open and the leakage spec is assuming no base current or in other words the collector leakage current is being gained up by that very temperature sensitive transistor beta. Very few if any spec sheets give the spec you are actually interested in, which is when DRIVEN off what does it still conduct.
Dan