It seems to be the case in hb-view too.
Here is the priority order for "0" in FiraCode
hb-view --features='cv11,zero' FiraCode-Regular.ttf '0000' -O png >
test.png gives
zero
hb-view --features='cv11,cv12' FiraCode-Regular.ttf '0000' -O png >
test.png gives
cv11
hb-view --features='cv12,cv13' FiraCode-Regular.ttf '0000' -O png >
test.png gives
cv12

It was also in the case of the Iosevka font file.