Blame the newer drivers. Beginning with the 26.5.1 driver version, AMD no longer supports OpenCL, instead switching completely to ROCm support. Version 26.3.1 was the last driver with OpenCL support. After updating to 26.5.1, OpenCL will be disabled on any Radeon GPU which supports these newer driver versions-a situation that cannot be fixed with a simple reversion to an older driver version. That means that if Premiere even ran at all on any AMD GPU-equipped system with these newer drivers, the renderer would have been locked to the software-only mode (in Premiere versions 25.5 and earlier).
What gives is that ROCm 6 support had been in the earlier drivers, but 26.5.1 introduced ROCm 7 support, which is breaking compatibility with legacy but still supported apps.
To regain OpenCL support, you must download the AMD Cleanup Utility from the AMD Web site, and run it in Windows Safe Mode. Then, after running that and when Windows reboots into normal mode, please install 26.3.1 or earlier.