That really looks like a bug (and yes I was able to reproduce this behavior). Could you contact Esri support and notify them?
There is a way to change the outline of all the polygons which seems to work (without editing the json of the element). In the symbol editor there is a button (see red outline in image below) that allows to change all the symbols (sorry for the Spanish interface):
Change the outline to the width, transparency, color, etc you want to assign:
... and click OK. It will assign it to all the symbols:
Be aware, if you change a single symbol afterwards (for instance the fill color without touching the outline symbol settings, all other outline will reset to the default again. So you should start with the fill colors and end with the outlines.