Thank you for the detailed description of the issue. As you pointed out the core issue is the simulation stopping before the expected breakdown. it's hard to troubleshoot without inspecting the file (which is unfortunately not allowed on the forum). However, one thing I notice based on your screenshot is that you are running a 2D simulation. Please note that this will not accurately capture the circular geometry of your photodiode, so a 3D simulation is needed. Hopefully, this is causing the unexpected IV behavior. Of course, running 3D simulation is more time consuming and might cause more convergence errors compared to 2D.
If switching to 3D doesn't resolve your issue, I recommend inspecting the carrier density and electric field results. This can be insightful at finding more information about the behavior of the device and in some cases helps you find the necessary updates to your model.
The general guidelines for resolving troubleshooting issues can also be helpful. I suspect you might have already checked this out but just in case: https://optics.ansys.com/hc/en-us/articles/8355464963987-Troubleshooting-convergence-errors-in-CHARGE