With BetaFlight on the same rig, same setup, with LiPo plugged in and USB connected to the Configurator -- Taranis discovers only the first four sensors from the XSR.
With the USB disconnected, Taranis discovers 13 sensors in total. 1 less than dRonin.
With BetaFlight, I notice that the asterisks on the Taranis Telemetry page indicting sensor updates flash much faster than they do in dRonin. And the sensor updates never freeze like they do in dRonin. Sensor discovery works repeatedly and predictably. No issues whatsoever.
Going back to the dRonin Release-20160720 everything behaves as described previously: sensors get discovered only after the rig has been unplugged for a little while; all FC sensors stop refreshing within a minute. It seems like the XSR loses the SPort connection altogether.
Another note -- when sensor discovery does work with dRonin, all the sensors get discovered in random order. Is it feasible to apply a sort routine to the sensors before sending them over the SPort? The sort order of the sensors coming from BetaFlight was much more sensible.
Next, I'll see if I can reproduce this issue in dRonin with the X4RSB Non-EU Version 150630 in place of the XSR.