Sound card plays the modulator channel after set to modulate multiple channels

[Fingercomp]

Code to reproduce the issue:

local s = require("component").sound

s.setWave(1, s.modes.square)
s.setWave(3, s.modes.sine)

s.setFrequency(1, 440)
s.setFrequency(3, 220)

s.setVolume(1, 1)
s.setVolume(3, 1)

s.setTotalVolume(1)

s.setFM(1, 3, 220)
-- ch3 now modulates ch1

s.open(1)
s.open(3)
s.delay(1000)

s.setFM(2, 3, 220)
-- ch3 now modulates both ch1 and ch2
s.resetFM(2)
-- the modulator for ch2 is now removed
-- ch3 should only be modulating ch1

s.delay(1000)
-- expected behavior: no change is observed
-- actual behavior: the sound clearly changes

s.close(1)
s.close(3)

s.process()

As far as I understand, whether a channel is mixed into the output is controlled by the isFreqMod and isAmpMod flags. They are set when the channel is set as a modulator for another channel and reset when the modulation is removed. In the code above, channel 3 is modulating two channels. The modulation of one of the carrier channels is then removed, which resets the isFreqMod flag even though the first channel still has the channel 3 set as its modulator.