Fix scores sometimes not being re-standardised correctly

osu.Game/Online/API/Requests/Responses/APILegacyScoreInfo.cs

@@ -38,6 +38,7 @@ namespace osu.Game.Online.API.Requests.Responses
                 Rank = Rank,
                 Ruleset = ruleset,
                 Mods = mods,
+                IsLegacyScore = true
             };
 
             if (Statistics != null)

osu.Game/Scoring/ScoreInfo.cs

@@ -185,6 +185,34 @@ namespace osu.Game.Scoring
         [JsonProperty("position")]
         public int? Position { get; set; }
 
+        private bool isLegacyScore;
+
+        /// <summary>
+        /// Whether this <see cref="ScoreInfo"/> represents a legacy (osu!stable) score.
+        /// </summary>
+        [JsonIgnore]
+        [NotMapped]
+        public bool IsLegacyScore
+        {
+            get
+            {
+                if (isLegacyScore)
+                    return true;
+
+                // The above check will catch legacy online scores that have an appropriate UserString + UserId.
+                // For non-online scores such as those imported in, a heuristic is used based on the following table:
+                //
+                //       Mode      | UserString | UserId
+                // --------------- | ---------- | ---------
+                // stable          | <username> | 1
+                // lazer           | <username> | <userid>
+                // lazer (offline) | Guest      | 1
+
+                return ID > 0 && UserID == 1 && UserString != "Guest";
+            }
+            set => isLegacyScore = value;
+        }
+
         public IEnumerable<(HitResult result, int count, int? maxCount)> GetStatisticsForDisplay()
         {
             foreach (var key in OrderAttributeUtils.GetValuesInOrder<HitResult>())

osu.Game/Scoring/ScoreManager.cs

@@ -10,6 +10,7 @@ using System.Threading;
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore;
 using osu.Framework.Bindables;
+using osu.Framework.Extensions;
 using osu.Framework.Logging;
 using osu.Framework.Platform;
 using osu.Game.Beatmaps;
@@ -149,23 +150,38 @@ namespace osu.Game.Scoring
                     return;
                 }
 
-                int? beatmapMaxCombo = score.Beatmap.MaxCombo;
+                int beatmapMaxCombo;
 
-                if (beatmapMaxCombo == null)
+                if (score.IsLegacyScore)
                 {
-                    if (score.Beatmap.ID == 0 || difficulties == null)
+                    // This score is guaranteed to be an osu!stable score.
+                    // The combo must be determined through either the beatmap's max combo value or the difficulty calculator, as lazer's scoring has changed and the score statistics cannot be used.
+                    if (score.Beatmap.MaxCombo == null)
                     {
-                        // We don't have enough information (max combo) to compute the score, so let's use the provided score.
+                        if (score.Beatmap.ID == 0 || difficulties == null)
-                        Value = score.TotalScore;
+                        {
+                            // We don't have enough information (max combo) to compute the score, so use the provided score.
+                            Value = score.TotalScore;
+                            return;
+                        }
+
+                        // We can compute the max combo locally after the async beatmap difficulty computation.
+                        difficultyBindable = difficulties().GetBindableDifficulty(score.Beatmap, score.Ruleset, score.Mods, (difficultyCancellationSource = new CancellationTokenSource()).Token);
+                        difficultyBindable.BindValueChanged(d => updateScore(d.NewValue.MaxCombo), true);
+
                         return;
                     }
 
-                    // We can compute the max combo locally after the async beatmap difficulty computation.
+                    beatmapMaxCombo = score.Beatmap.MaxCombo.Value;
-                    difficultyBindable = difficulties().GetBindableDifficulty(score.Beatmap, score.Ruleset, score.Mods, (difficultyCancellationSource = new CancellationTokenSource()).Token);
-                    difficultyBindable.BindValueChanged(d => updateScore(d.NewValue.MaxCombo), true);
                 }
                 else
-                    updateScore(beatmapMaxCombo.Value);
+                {
+                    // This score is guaranteed to be an osu!lazer score.
+                    // The combo must be determined through the score's statistics, as both the beatmap's max combo and the difficulty calculator will provide osu!stable combo values.
+                    beatmapMaxCombo = Enum.GetValues(typeof(HitResult)).OfType<HitResult>().Where(r => r.AffectsCombo()).Select(r => score.Statistics.GetOrDefault(r)).Sum();
+                }
+
+                updateScore(beatmapMaxCombo);
             }
 
             private void updateScore(int beatmapMaxCombo)