The following topic was closed without a solution being suggested:
In case someone comes looking for help, the new nvstreammux will slow down to the framerate of the slowest source when sync-inputs=1. This means that if your network is slow or a frames are dropped intermittently from a source, the other sources will suffer while waiting for the slowest source to respond.
To get around this, the failing source must be unlinked from the nvstreammux: unlink_element_from_streammux_sink_pad() so that the others can continue streaming.