Here's the firmware notes (version 3, the later updates do not change this as far as I can see). If your camera isn't doing this I can't think why.
*Some functions are restricted when the"ELECTRONIC SHUTTER" or "MECHANICAL+ELECTRONIC" is selected.
The shutter speed and sensitivity are restricted to values of 1/32000-1 sec and ISO 6400-200 respectively.
Regardless of the option selected, the mechanical shutter is used for motion panoramas.
In the burst mode, focus and exposure are fixed at the values for the first shot in each burst.
In the range where the electronic shutter works, the long exposure noise reduction doesn't work.
When the "ES ELECTRONIC SHUTTER" is selected, the FLASH MODE is fixed to the "SUPPRESSED FLASH".
When the "MS MECHANICAL+ES ELECTRONIC" is selected, the mechanical shutter is automatically selected for a TTL flash to fire if flash firing is required. With a non-TTL flash, the flash doesn't fire in the mode. So use the flash in the "MS MECHANICAL SHUTTER" mode.