When a user release his PTT in a TG the prolonged time for LED alert for that TG seems to stop for the rest of that conversation.
I guess the "while the voice goes on" are actually "while the voice goes on from the current user"
I suggest to instead make use of the TG delay timer to control the LED. When TG delay timer are active the LED are active.
Also there's no prolonged LED alert time active for analog channels, that then also would need the channel delay timer to be used.