When using a JSON list that consists of only MusicBrainzId's, the list is being filtered. This appears to fix the issue, or it may have just broke the check, please check.

src/NzbDrone.Core/ImportLists/FetchAndParseImportListService.cs

@@ -135,7 +135,7 @@ namespace NzbDrone.Core.ImportLists
 
             Task.WaitAll(taskList.ToArray());
 
-            result = result.DistinctBy(r => new { r.Artist, r.Album }).ToList();
+            result = result.DistinctBy(r => new { r.Artist, r.Album, r.ArtistMusicBrainzId }).ToList();
 
             return result;
         }

src/NzbDrone.Core/ImportLists/ImportListBase.cs

@@ -64,7 +64,7 @@ namespace NzbDrone.Core.ImportLists
 
         protected virtual IList<ImportListItemInfo> CleanupListItems(IEnumerable<ImportListItemInfo> releases)
         {
-            var result = releases.DistinctBy(r => new { r.Artist, r.Album }).ToList();
+            var result = releases.DistinctBy(r => new { r.Artist, r.Album, r.ArtistMusicBrainzId }).ToList();
 
             result.ForEach(c =>
             {