There is no proper way to have reverb tail off naturally, other than with weird workarounds that are very clunky to work with. It's one thing that reverb applied directly to a clip or even on the track ends as soon as the clip ends (and therefore doesn't function as a reverb), but when it's sent to a submix track the reverb should tail off naturally like it does with literally every other programme.
Premiere Pro really should have the proper tools to finish audio properly without workarounds.
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