SourceDetectionTask can silently corrupt the provided exposure by fitting and subtracting an invalid background model that that results in the image being all nans. I believe the task should realize that the background model is invalid and raise an exception in this situation.
I first saw the problem with an image that had several hundred counts of background. Detection detected a single source and the results were as described. I have attached code demonstrating the issue. Setup `meas_algorithms` and run `showBackgroundBug.py` to see the problem.