Internally, there should be  6-bits register width (0-63) to represent the amplitude, but for some reason only 10 steps are used in the attack phase.

Source 

The OPLL series has a function that automatically damp the previous releasing note when the next note is Key-ON during the release phase of previous note.

(Indicated as "DP" in the left figure.)

Source 

However, when there is a difference in the time between the modulator and the carrier being completely damped, it causes an unpleasant noise at the beginning of the next note.

Detail 

When percussive tone is selected (EGT=0), OPLL transitions to the release phase at a rate equivalent to RR=7 from the Key-OFF point. To obtain the same envelope waveform in the OPL series, write EGT=1 and RR=7 at the same time as Key-OFF. (Of course, the RR can be any value you desired.)

Source 

Even when a note is Key-OFFed, the modulator's envelope generator does not transition to the release phase and keeps its sustain phase behavior after Key-OFF.

Presumably, this behavior is a glitch, which is causing a misalignment of the damp timing between the modulator and carrier.

Source