Hi @TonghuiMing - thanks for the response
My expectation was that the zoom to option would zoom to the full extent of the selected polygon, and wouldn't keep zooming if you pressed zoom to again. Chatting to some others in the office, this was also their expectation based on past experience.
I have gone back and looked in some other apps to compare the behaviour and am getting mixed results.
I have two layers in another app (an AGOL dashboard with a map widget) - using the popup zoom to option on one layer zooms to full extent of the selected feature (as expected), but the other layer has the same behaviour as my original post.
In another app (a Portal WAB), I'm also getting different behaviour on different layers. This app has the attribute table activated, and interestingly, on the layers that have the same behaviour as my original post, the zoom to option in the attribute table doesn't work. I.e. I select a feature in the attribute table, press zoom to and the map extent doesn't change at all. Seems like it might be related?