If you have ever been shooting with an on-camera V860II acting as a (radio) Master controller, in Gr (group) mode with manual power levels set, and have inexplicably seen all your groups' power settings suddenly reset to the same value after switching modes, this post may be of interest.
If you haven't, this post will be seriously tedious and if you read it anyway... lord help you.
Summary: In the above scenario, pressing the Mode button to cycle through the various modes can result in all groups reset to the last manual power level that Group A had been set to in Radio Master - Group mode. This is NOT a good thing, especially in high-pressure situations.
To see the bug:
Start from the default mode, with the large "ETTL" showing, upper-left.
Press the wireless mode selector button (far right) three times to select the Radio Master wireless mode.
The screen should show an antenna icon and a small "ETTL" at upper-left, and CH X MASTER RATIO OFF in the middle of the screen. Let's call this Radio Master - ETTL mode.
Press the mode button (far left). The screen will now show the antenna icon and "M" at upper-left. Let's call this Radio Master - M mode. At the center of the screen there will be CH X MASTER RATIO OFF and below that, the word "ALL" framed by a box and "1/XXX" showing a power setting between 1/128 and 1/1. (This "ALL" is relevant in the eventual workaround to this bug)
Press the mode button again. The screen will now show the antenna icon and "MULTI" at upper-left. Let's call this Radio Master - Multi mode.
Press the mode button again. The screen will now show the antenna icon and "Gr" at upper-left. Let's call this Radio Master - Group mode.
Using the appropriate Mode soft-key, rotating ring and Set button, set a few groups to some different Manual power settings, e.g. A to 1/16, B to 1/32 and C to 1/4.
Now, press the Mode physical button (far left). You'll be in Radio Master - ETTL.
Press Mode again. You'll be in Radio Master - M mode" By default, at this point you'll see the word "ALL" in a box, and a power level displayed.
Press Mode again to get to Radio Master - Gr mode. Now, hope that you wrote down your Group power settings on a post-it, cuz this is the moment you are screwed if you had previously set different power levels, because those power levels are now gone into the ether. All groups will now be set to the power level you saw a moment ago next to the word ALL as you passed through M mode on the way to Gr mode.
Test your ability to make small talk with your subject(s) as you try to recall which group was set to what, and punch / dial them back in to the stupid V860II UI.
Get into Radio Master - M mode. Press the RATIO button so that it no longer displays "ALL" but instead shows either separate A and B values, or A / B / C values.
*note that a factory reset / firmware update will reset this screen to the default "ALL" - and the Gr-mode reset problem will be back.*
If, when changing modes, you simply press the mode button fast enough - I'd say less than 1/4 second - to go "past" the Radio Master - M screen, the groups will not be reset.
This eccentricity is why it took me so damn long to figure out why I would see this problem *sometimes*, but not always. Often I would be shooting with all-manual off-camera flashes controlled by the on-camera Gr-mode V860II, then something would happen that required me to stop the off-camera wireless control ("hey, can you come into this other room to take a picture of X"). When I would go back into Gr mode sometimes I would press the Mode button quickly enough that the group settings were maintained. But I had no idea why most of the time they would be Gone.
I'm happy I figured this out tonight since I can now avoid the problem. I was sometimes so stressed by this issue that I would keep a second V860II on me that I would swap onto the camera rather than leave Gr mode, if I thought I might need to switch between wireless and straight flash in a hurry. Felt like an idiot every time I did it, but it was the only way I knew my settings would be intact.
Anyway, I thought I'd type all this out in case someone else somewhere is banging their head into the same problem.