--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -1095,17 +1095,22 @@ void MediaDecoderStateMachine::Recompute
   } else if (mEstimatedDuration.Ref().isSome()) {
     duration = mEstimatedDuration.Ref().ref();
   } else if (mInfo.mMetadataDuration.isSome()) {
     duration = mInfo.mMetadataDuration.ref();
   } else {
     return;
   }
 
-  if (duration < mObservedDuration.Ref()) {
+  // Only adjust the duration when an explicit duration isn't set (MSE).
+  // The duration is always exactly known with MSE and there's no need to adjust
+  // it based on what may have been seen in the past; in particular as this data
+  // may no longer exist such as when the mediasource duration was reduced.
+  if (mExplicitDuration.Ref().isNothing() &&
+      duration < mObservedDuration.Ref()) {
     duration = mObservedDuration;
   }
 
   MOZ_ASSERT(duration.ToMicroseconds() >= 0);
   mDuration = Some(duration);
 }
 
 void