Some info that may be helpful (may already be known).
Looks like the game endings stats tracker is reset to zero every time the game is complete, therefore the stats tracker will never get above 1/5 (20%) for new gamers.
I reset my app data in windows then loaded up and ran through my five end games:
synedron [terra, poly], anu, nj, pp
Each time I got this same UnityLog entry:
[INFO] 28410 (491.639): Set player stat “GameEndingsAccomplished” to 0
UnityTools.LogFormatter:LogFormat(LogType, Object, String, Object[])
UnityEngine.Logger:Log(LogType, Object, Object)
UnityEngine.Debug:Log(Object, Object)
Base.Utils.Dbg.Diag:LogInternal(LogCategory, String, Object)
Base.Utils.Dbg.Diag:Log(LogCategory, String, Object)
Base.Platforms.Generic.PlatformPlayerStatsGeneric:SetStatInt(String, Int32)
PhoenixPoint.Tactical.Achievements.TacPlayerStatsTracker:OnGameOverEvent(TacticalLevelController)
System.Xml.OnRemoveWriter:Invoke(XmlRawWriter)
PhoenixPoint.Tactical.Levels.TacticalLevelController:GameOver()
PhoenixPoint.Tactical.Levels.GameOverConditions.d__13:MoveNext()
Base.Core.Updateable:Call()
Base.Core.TimingScheduler:CallUpdateable(TimeUnit, Updateable)
Base.Core.TimingScheduler:Update(UpdatePhase)
Base.Core.d__5:MoveNext()
UnityEngine.SetupCoroutine:InvokeMoveNext(IEnumerator, IntPtr)
…
[INFO] 28411 (491.698): [ACHIEVEMENT] AllEndings 1/5 (20/100
UnityTools.LogFormatter:LogFormat(LogType, Object, String, Object[])
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:Log(Object)
Base.Achievements.AchievementTracker:StoreAchievements()
This same log entry was repeated each time I finished the game with each ending.
Hope this helps a little.