Windows Task automatically wakes my computer, but doesn't put it back to sleep after

Windows 10 is by default configured to timeout and return to sleep two minutes after a system unattended wake up.

To verify these settings, start regedit and navigate to the key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\PowerSettings\238C9FA8-0AAD-41ED-83F4-97BE242C8F20\7bc4a2f9-d8fc-4469-b07b-33eb785aaca0\DefaultPowerSchemeValues.

You will see here three entries:

381b4222-f694-41f0-9685-ff5bb260df2e    Balanced
8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c    High performance
a1841308-3541-4fab-bc81-f71556f20b4a    Power saver

Click on the entry corresponding to your power plan and verify the settings for the items ACSettingIndex for when plugged in and DCSettingIndex for when on battery.

Both items should have by default the value of "0x00000078 (120)". If they don't show this value, then set them as above.

For more information and methods, see the articles:

• How to Change System Unattended Sleep Timeout in Windows 10
• Windows 7: Sleep Return Timeout for Unattended Wake Up

I solved this by selecting the "Balanced" power plan in Control Panel > All Control Panel Items > Power Options. My theory, after reading harrymc's reply, is that the custom power plan I had created, didn't have an entry in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\PowerSettings\238C9FA8-0AAD-41ED-83F4-97BE242C8F20\7bc4a2f9-d8fc-4469-b07b-33eb785aaca0\DefaultPowerSchemeValues. If that doesn't work, try clicking "change plan settings" and "restore default settings for this plan". Then of course make sure this plan is selected.