VK_NV_raytracing: Update scratch size of compacted AS returns 0

If i create a compacted bottom AS with the ALLOW_UPDATE flag, querying for MEMORY_REQUIREMENTS_TYPE_UPDATE_SCRATCH returns a size of 0 with non-zero alignment and type bits. But when i use this AS as the target of an update build (with the source having same flags and compacted size, which was copied to before), some amount of scratch memory is written to by the GPU. It will overwrite my own data if i do not leave some space. The amount seems to be quite small, like 1 float.

Workaround seems easy, simply adjust the returned size if it is 0, changing it to 4 seems to be enough with my hardware/driver/data, but you might want to use 64k to be more save. But i would still like a fix that returns the actual size.

My system:
Win7 x64
RTX 2070
Driver 417.63
LunarG SDK
Vulkan Header 1.1.94
MSVC 2017 15.9.5


After some more testing, the required size depends on more flags. With the FAST_TRACE flag, the update step requires a lot more memory. Adjusting the compacted build size to the next 64kb border seems to work. Since the size depends on the flags a fix would be really appreciated.