Panasonic FZ1000: Flash FP high-speed sync oddities, 1/16000s mechanical shutter?

Because the Panasonic LUMIX FZ1000 uses a leaf shutter, there is no maximum sync speed; the flash can be used throughout the entire range of mechanical shutter speeds, as high as 1/4000s (1/3200s unless stopped down at 32mm equiv. 135 or longer). Ordinarily, this means that FP high-speed sync isn’t normally necessary. Nonetheless, FP sync can be enabled on a shoe-mounted Four Thirds TTL flashgun (such as the Panasonic DMW-FL360L) and the flash will fire continuously throughout the exposure, just as it would on a camera with a focal-plane shutter. This is evident in the flash no longer freezing very fast motion when it would in normal mode.

However, I’ve noticed a number of oddities with the flash when it’s in FP sync mode (all shots taken in manual exposure mode):

• The flash may fail to sync properly when the aperture is wide-open at focal lengths 33mm (equiv. 135) and below; while the flash fires, the picture may be exposed as though the flash wasn’t fired during the exposure. This might have to do with the 1/3200 mechanical shutter speed wide-open at 32mm and above. (This does seem to indicate that the flash is operating as though the camera has a focal-plane shutter with a sync speed of 1/125s, possibly a dummy value communicated to the flash for compatibility purposes.)
• More significantly, with Shutter Type (Rec menu page 4) set to Auto, the camera allows the use of the mechanical shutter at speeds as high as 1/16000s, speeds that are normally not attainable without using the electronic shutter. The camera is in fact using the mechanical shutter; the leaf shutter click can be heard when taking a picture, and there is no “E with shutter button” symbol on the screen indicating use of the electronic shutter. The flash fires, and shots taken in this mode are in fact exposed correctly as long as the subject is within range of the flash. These shutter speeds appear to be real: if the subject is out of range of the flash, the image becomes progressively underexposed as the shutter speed increases. Furthermore, stopping down the aperture does affect the exposure and depth of field as it normally does; faster shutter speeds do not result in an increase in depth of field (which would be expected if the leaf shutter can’t open fully at high speeds).

If the mechanical shutter can only operate at 1/4000s, how is it possible for it to reach 1/16000s when using a flashgun in FP high-speed sync mode? Is this the result of some sort of firmware glitch? (It may be worth noting that the camera always uses the leaf shutter in conjunction with an “electronic first curtain”; there is no audible sound when a long exposure begins, only when the exposure ends, and the shutter immediately reopens to resume live view unless long-exposure noise reduction is activated.)