[OnePlus 8T Questions & Answers] MarvMan²:
Thanks for your answer. Yes i got root. The app Naptime says the phone is in deep doze / deep sleep but other apps say it's not and the battery drain is 20% in 10h.

Edit: The high battery drain shows the problem is still present.

EDIT2: Maybe this helps:

OnePlus8T:/ $ dumpsys power
POWER MANAGER (dumpsys power)

Power Manager State:
Settings power_manager_constants:
no_cached_wake_locks=true
mDirty=0x0
mWakefulness=Asleep
mWakefulnessChanging=false
mIsPowered=false
mPlugType=0
mBatteryLevel=86
mBatteryLevelWhenDreamStarted=0
mDockState=0
mStayOn=false
mProximityPositive=false
mBootCompleted=true
mSystemReady=true
mHalAutoSuspendModeEnabled=true
mHalInteractiveModeEnabled=false
mWakeLockSummary=0x1
mNotifyLongScheduled=+7s242ms
mNotifyLongDispatched=-52s843ms
mNotifyLongNextCheck=(none)
mUserActivitySummary=0x0
mRequestWaitForNegativeProximity=false
mSandmanScheduled=false
mSandmanSummoned=false
mBatteryLevelLow=false
mLightDeviceIdleMode=false
mDeviceIdleMode=false
mDeviceIdleWhitelist=[1000, 1001, 2000, 10085, 10093, 10096, 10136, 10142, 10166, 10181, 10191, 10193, 10204, 10206, 10218, 10234, 10260, 10338, 10339, 10341]
mDeviceIdleTempWhitelist=[]
mLastWakeTime=68485298 (720216 ms ago)
mLastSleepTime=69054615 (150899 ms ago)
mLastSleepReason=timeout
mLastUserActivityTime=68934615 (270899 ms ago)
mLastUserActivityTimeNoChangeLights=68763480 (442034 ms ago)
mLastInteractivePowerHintTime=69059697 (145817 ms ago)
mLastScreenBrightnessBoostTime=0 (69205515 ms ago)
mScreenBrightnessBoostInProgress=false
mDisplayReady=true
mHoldingWakeLockSuspendBlocker=true
mHoldingDisplaySuspendBlocker=false

Settings and Configuration:
mDecoupleHalAutoSuspendModeFromDisplayConfig=true
mDecoupleHalInteractiveModeFromDisplayConfig=true
mWakeUpWhenPluggedOrUnpluggedConfig=true
mWakeUpWhenPluggedOrUnpluggedInTheaterModeConfig=false
mTheaterModeEnabled=false
mSuspendWhenScreenOffDueToProximityConfig=false
mDreamsSupportedConfig=true
mDreamsEnabledByDefaultConfig=true
mDreamsActivatedOnSleepByDefaultConfig=false
mDreamsActivatedOnDockByDefaultConfig=true
mDreamsEnabledOnBatteryConfig=false
mDreamsBatteryLevelMinimumWhenPoweredConfig=-1
mDreamsBatteryLevelMinimumWhenNotPoweredConfig=15
mDreamsBatteryLevelDrainCutoffConfig=5
mDreamsEnabledSetting=true
mDreamsActivateOnSleepSetting=false
mDreamsActivateOnDockSetting=true
mDozeAfterScreenOff=false
mMinimumScreenOffTimeoutConfig=10000
mMaximumScreenDimDurationConfig=15000
mMaximumScreenDimRatioConfig=0.25
mAttentiveTimeoutConfig=-1
mAttentiveTimeoutSetting=-1
mAttentiveWarningDurationConfig=30000
mScreenOffTimeoutSetting=120000
mSleepTimeoutSetting=-1
mMaximumScreenOffTimeoutFromDeviceAdmin=9223372036854775807 (enforced=false)
mStayOnWhilePluggedInSetting=0
mScreenBrightnessModeSetting=1
mScreenBrightnessOverrideFromWindowManager=NaN
mUserActivityTimeoutOverrideFromWindowManager=10000
mUserInactiveOverrideFromWindowManager=false
mDozeScreenStateOverrideFromDreamManager=0
mDrawWakeLockOverrideFromSidekick=false
mDozeScreenBrightnessOverrideFromDreamManager=-1
mScreenBrightnessSettingMinimumFloat=0.001954079
mScreenBrightnessSettingMaximumFloat=1.0
mScreenBrightnessSettingDefaultFloat=0.45882353
mDoubleTapWakeEnabled=false
mIsVrModeEnabled=false
mForegroundProfile=0
mUserId=0

Attentive timeout: -1 ms
Sleep timeout: -1 ms
Screen off timeout: 10000 ms
Screen dim duration: 2500 ms