The L2 EVSE and the LEAF's internal charger limits the power into the Battery Pack
when doing L2 charging. The heating just uses energy from the Battery Pack,
independent of the charging process. Since a Chademo QC (some call it L3)
charging session can add energy to the Battery Pack a LOT faster, any energy
being used for heating (while QC is in progress) is a smaller fraction of the
incoming (charging) energy, and essentially goes directly from the QC to
the heater, with less going into the Battery Pack.
It is possible that the QC controller in the LEAF is monitoring the current
going into the Battery Pack, and it could request more current from the
QC device, to make up for current used by the heater. This might vary
between model years, and in different firmware versions. Apparently,
you observed this happening ... in what year LEAF?