Hi Nick,
You assume correctly. In soundfonts, stereo samples are considered as two mono samples whose pan is -50 for left and +50 for right. Combined, this is exactly the same than listening to a stereo sample made of these two channels.
The left panning for the lowest notes and the right panning for the highest notes are thus registered directly in the samples (this is not a good practice). You can try to revert it by applying an opposite pan with this
panning tool.
Regards,
Davy