I don't own or use a NSB stagebox, so I can't replicate your case. But maybe this can be helpful: "StudioLive_Series_III_AVB_Networking_Guide_EN_11092018", section 2.7.4 "Clocking a PreSonus AVB Network":
"NSB-Series stage boxes are designed to provide both remote inputs and remote outputs for StudioLive Series III mixers. Like the EarMix 16M, the NSB-series stage boxes will default to external clock and sync to a StudioLive Series III mixer as soon as AVB sends are routed to their outputs.
It is important to note that these AVB sends must be routed whether they are needed or not. Let’s look at the common example of a StudioLive 16 console mixer with two NSB 8.8 stage boxes. In some situations, the eight outputs on the second NSB 8.8 stage box may not be needed to pass audio. In this case, routing an AVB output stream from the mixer to the stage box would still be required for proper clocking."
The last sentence might be of special importance. The EarMixers also react to missing AVB sends with drop outs.
You find the manual on the download page under "Support"on the PreSonus web site.