From your description, I guess it is caused by the post-processing of SGIE(car license plate detection).
Try adjusting the pre-cluster-threshold
https://docs.nvidia.com/metropolis/deepstream/dev-guide/text/DS_plugin_gst-nvinfer.html#id3
It’s open source，refer this file ./opt/nvidia/deepstream/deepstream/sources/libs/nvdsinfer/nvdsinfer_context_impl_output_parsing.cpp