As the title says, all display drivers after 430.86 seem to be causing Iray CPU fallback on both RTX and non-RTX cards, regardless of the amount of memory used by the scene. The only difference is that the newer the drivers the faster they fallback to CPU. With the latest 441.28 studio driver I just switch to Iray preview on an empty scene and it falls back to CPU.
Since it worked in 430.86 I am going to assume it is a driver regression. There is no proper channel to report this issue to NVIDIA as a user so because Iray and OptiX are connected I am posting here hoping that someone from NVIDIA OptiX team will pick it up and investigate.
2019-11-23 16:47:04.409 Iray [INFO] - IRAY:RENDER :: 1.15 IRAY rend info : Using OptiX version 6.1.2
2019-11-23 16:47:04.414 Iray [INFO] - IRAY:RENDER :: 1.15 IRAY rend info : Initializing OptiX for CUDA device 0
2019-11-23 16:47:04.478 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [ERROR] - IRAY:RENDER :: 1.15 IRAY rend error: "buildInputs[0].instanceArray", "numInstances" is non-zero, but "instances" is null
2019-11-23 16:47:04.478 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [ERROR] - IRAY:RENDER :: 1.15 IRAY rend error: optixAccelBuild( context,nullptr, &opt,&inst_input,1, get_address(m_d_scratch),tlas_size.tempSizeInBytes, get_address(m_tlas.data),tlas_size.outputSizeInBytes, &m_tlas.traversable, &bbox_desc,1) failed: Invalid value
2019-11-23 16:47:04.479 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [ERROR] - IRAY:RENDER :: 1.10 IRAY rend error: CUDA device 0 (GeForce RTX 2080 Ti): Scene setup failed
2019-11-23 16:47:04.479 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [ERROR] - IRAY:RENDER :: 1.10 IRAY rend error: CUDA device 0 (GeForce RTX 2080 Ti): Device failed while rendering
2019-11-23 16:47:04.479 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [WARNING] - IRAY:RENDER :: 1.10 IRAY rend warn : All available GPUs failed.
2019-11-23 16:47:04.479 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [WARNING] - IRAY:RENDER :: 1.10 IRAY rend warn : No devices activated. Enabling CPU fallback.
2019-11-23 16:47:04.479 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [WARNING] - IRAY:RENDER :: 1.10 IRAY rend warn : Re-rendering iteration because of device failure
2019-11-23 16:47:04.479 WARNING: ..\..\..\..\..\src\pluginsource\DzIrayRender\dzneuraymgr.cpp(304): Iray [ERROR] - IRAY:RENDER :: 1.10 IRAY rend error: All workers failed: aborting render
DAZ Studio version is 4.12.1.16 Public Beta. Iray version is 5.0, build 317500.5529 n, 14 Sep 2019, nt-x86-64-vc14. Hardware is RTX 2080 Ti.