OnePlus 13 OOS 15: Private DNS keeps failing during overnight Doze and causes high battery drain, what to disable?

I’m using a OnePlus 13 on OxygenOS 15 with the same Wi-Fi network and DNS setup that I previously used on my OnePlus 7T without any problems.

I use Private DNS → dns.adguard.com.

During normal daytime use, everything works perfectly. However, when I leave Wi-Fi on overnight with the screen off, I consistently get this notification:

“Failed to connect to the private DNS server. Please turn off Private DNS to connect to the internet.”

The strange part is that as soon as I wake/unlock the phone, DNS starts working again immediately.

Battery drain is also significantly higher overnight. I have seen around 10–20% overnight drain when this happens.

I tested this with accuBattery and noticed something interesting: during screen-off time, the phone appears to remain “Held awake” rather than entering the reported deep-sleep state.

I’m trying to determine whether this is:

A Google/HeyTap/OnePlus service preventing proper Doze

Something related to Sleep Standby Optimization

The same Wi-Fi and DNS setup did not cause this behavior on my OnePlus 7T.

Has anyone with a OnePlus 13/OxygenOS 15 experienced the same thing? If so, what specifically fixed it?

I’m particularly interested in whether disabling Sleep Standby Optimization, changing Private DNS, or disabling a particular system package actually fixes the problem.

I don't want to simply disable Private DNS because AdGuard DNS itself works perfectly when the phone is awake. I want to identify what is happening specifically during Doze/overnight standby.