Just for anybody looking for info on this, I spent some time debugging and tracked it down to a bug in UWidgetComponent in finding local player owners for remote pawns from the listen server. I looked for the bug tracker submission above but couldn’t find it, so I made my own submission and included the repro and all the info I had regarding the cause.
The issue seems to be work-around-able by subclassing UWidgetComponent and overriding the owner player with the first available local player: this info used internally for sorting screen widgets and doesn’t seem to have any other effect (aside from making the widget fail to add to screen for remote client pawns). Just leaving this here in case anybody else runs into the same problem.
After you’ve subclassed the component, just use it in place of your standard UWidgetComponent in your pawn blueprint. I haven’t tested extensively but this seems to work around the issue ok from my end.