Windows 10 keeps waking my monitor from sleep for no reason

The key distinction is whether the PC is asleep or only the display has switched off. Wake timers and device wake permissions control waking the PC; they do not necessarily prevent the display from turning on while Windows is still running.

Try this:

• Select Start > Power > Sleep instead of waiting for the screen timeout. If it stays asleep, focus on activity turning the display back on rather than sleep wake settings.
• If the PC actually wakes, open Command Prompt as administrator immediately afterward and run:

The first may identify the last wake source; the second lists devices still allowed to wake the computer. Check for a network adapter or another device you have not disabled. A blank or unknown wake source does not rule out a genuine wake.

• If only the display wakes, temporarily disconnect controllers, wireless receivers, docks and other unnecessary USB devices. A device can send input while Windows is awake even when its permission to wake the computer is disabled.
• Close media players, remote-access tools and peripheral utilities one at a time, then let the display time out again. This helps identify software triggering it.
• Watch what appears when the monitor lights up. If it only displays “No signal” and goes dark again, check the monitor’s automatic input-switching setting and test a direct video connection without a dock or adapter. That may be the monitor reacting to a connection change rather than Windows waking.

These checks should separate an actual system wake, unexpected input and a monitor connection issue before you change more power settings.