However, OP did say that it was working fine for 2 years when setup like this.
Maybe OP has a TV like mine … Sony TV is great with the USB port.
Seems to have enough power to keep RPi (1 and now 2) happy (no extra peripherals attached to RPi USB and connecting to LAN via Ethernet) and when using TV remote to switch off TV … the TV sends CEC display off info but keeps USB power on long enough for the RPi to perform orderly shutdown.
Check that OSMC/Kodi CEC settings are still set to tell RPi to shutdown when display turned off.
If the power from TV USB was marginal then it is possible that newer OSMC build needs more power to do stuff and means that RPi could suffer but modern RPi should show warning on screen.
(OP said that USB disk had own power supply)
So, while using TV USB remains a recommended “no” some might find, like me, that it works well.