Steps to repro:
1. Download the attached project "SponzaLWRP" and open "Sponza" scene;
2. Drag 'SponzaLightingDay' scene from the Project folder to the Hierarchy;
3. In the Hierarchy double-click on the SponzaLightingDay scene to make it the main scene;
4. In the Lighting window (Window>Rendering>Lighting) press Generate Lighting button;
5. Once "Baking..." is shown in the progress bar, wait for the progress bar to go beyond 50%;
6. In the Hierarchy select Directional Light gameobject and disable it;
7. Wait 2-5 seconds and then enable Directional light gameobject again;
8. Repeat steps 6-7 several times.

Expected result:
GPU baking proceeds and doesn't fall back to CPU Lightmapper.

Actual result:
Lightmapper switches to CPU PLM. The following errors are shown in the Console:
"[PathTracer] Convergence job with hash: dd2a59987ff0d75c63061d49e2abdd2c failed with exit code 3"
"OpenCL Error: 'clGetEventInfo(event, CL_EVENT_COMMAND_EXECUTION_STATUS, sizeof(eventExecutionStatus), &eventExecutionStatus, NULL)' returned -9999 (CL_UNKNOWN_ERROR)!
OpenCL Error. Falling back to CPU lightmapper. Error callback from context: CL_UNKNOWN_ERROR"

"OpenCL Error. Falling back to CPU lightmapper. Error callback from context: Unknown error executing clFlush on NVIDIA GeForce RTX 2080 Ti (Device 0)."

Regression introduced in Version 2021.2.0. See the attached video.

Notes:
- Reproducible in 2021.2.0b1, 2021.2.0a21, 2021.2.0a20, 2020.2.0a19;
- Not reproducible in 2021.1.12f1;
- Reproducible on Windows with NVIDIA GeForce RTX 2080Ti GPU;
- The fallback to CPU PLM doesn't happen when the light type is set to Mixed; only happens when the light type is Baked;
- The issue might not repro consistently. If it doesn't repro, try following:
- Varying the intervals between enabling/disabling the light;
- Re-enabling the light at the reflection probe baking stage (the last stage of baking);
- Setting the lightmap size in the Lighting window to 4096 and following the initial repro steps;
- Restarting Editor and repeating the initial repro steps.