Sounds like a HDMI “Handshake” issue between your WDTV & PHILIPS (sorry, can’t offer any help except what you discovered as a workaround to ‘force it’)
With the other issue of reverting back to 720p50hz i also have the same issue with my WDTV Live Hub
But i did a week long test that seemed to resolve this issue on my device.
I made sure my TV was in Standby (RED light … Not switched on)
Turned on my WDTV Live Hub … waited until it was fully booted up … then turned my TV On (GREEN light)
The WDTV Live Hub was still on the same resolution i selected the day before " 1080p 24hz"
(tried this 7 days in a row without it defaulting back to 720p50hz)
Of course … one day i forgot and turned my TV on 1st on HDMI1 and then powered on the WDTV Live Hub after the boot up … i checked the settings … and yep, it was back at 720p50hz
So, i can only assume the HMDI “Handshake” Between my WDTV & Panasonic TV defaults back to 720p50hz if i have the TV on during the boot process
Wonder if anyone else has tried this ?