According to my settings it can as both were enabled. I think the Lumix rep said High Stability mode supercedes Boost-IS so doesn't matter on/off.
I'm sure it does make a difference, at least in my testing it did.
I get the same issue what you had with IS-boost on, but not when it is set to off. For me it does make a difference.
Thats also partly what I meant I have to get better. You have a lot to check when shooting video, and I forget often a setting to check like:
-Boost-IS on/off
- E stabilization off/normal/high
- Aperture
- ISO
- Shutter-Angle
- ND-filter adjustment (for fixing exposure after setting the previous settings)
- FF / APS-C / Pixel
- Auto-focus mode including the people/person mode on/off and setting the small box correctly sometimes
- White balance (auto, Kelvin etc)
Almost every time I shoot Vlog, 25FPS, 4k 10bit 422 L.
Dometimes I do use 4K all-I or 5.8K Proresraw, but not in this trip.
In the end this is quite a lot for my 51-year old brain to keeep in check, at least on a trip together with my wife. On a controlled environment shoot like a Interview or greenscreen recording I found it much easier to do this.
But nice video (not looking at the IS-problem).
With Boost-IS off you could get very nice videos, the rest is for POST.