System sleeps immediately on remote power off command, despite blocking processes in PS (1 Viewer)

[rsbrux]

During the past couple of weeks, I have noticed that turning off my HTPC with the remote control sometimes puts the system to sleep immediately, even though:

1. I have set up PS in advanced/forced mode.
2. Processes are running which should prevent sleeping.

Logs are attached; the behaviour described above can be seen here in the TV Server log:

[2013-11-15 12:06:21,965] [Log    ] [PS StandbyWakeup] [DEBUG] - PS: SetStandbyMode(StandbyPrevented)
[2013-11-15 12:06:21,965] [Log    ] [PS StandbyWakeup] [DEBUG] - PS: System is busy and should not go to standby
[2013-11-15 12:06:25,455] [Log    ] [PowerEventThread] [DEBUG] - TV service PowerEventThread received WM_POWERBROADCAST 4
[2013-11-15 12:06:25,465] [Log    ] [PowerEventThread] [DEBUG] - OnPowerEvent: PowerStatus: Suspend
[2013-11-15 12:06:25,465] [Log    ] [PowerEventThread] [DEBUG] - OnPowerEventHandler: PowerStatus: Suspend
[2013-11-15 12:06:25,465] [Log    ] [PowerEventThread] [INFO ] - TvController.OnSuspend()

ISTR that this used to be OK (i.e. standby was effectively prevented in this case).
Could this be a result of:

1. Updating to MP1.5.0?
2. Some nefarious Windows update?
3. Something else I haven't thought of?

Any tips on how to correct this, or at least narrow it down, would be welcome.