The original thread above I put in JS API initially based on my first experiences with this bug. So it is ok to delete the first one. However, it appears to be more a function of a bug with ArcGIS Server 10.6.1 or what runs AGOL.
The bug: Add a polygon layer to ArcGIS in a web map. Enable its popup, and click it. The outline is way off and misaligned and warped in shape, as shown below. Esri TS has reproduced this, and assigned bug ID is #
Although it works fine in the 3.25 sandbox (as in this example). Thus it would seem AGOL introduced some sort of geometry simplification and/or projection issue with the July 2018 AGOL update.
One solution Esri could use to fix this, would be to simply re-draw the feature halo on a 'zoom-in' event, so that the geometry could be simplified at small scale and re-drawn with more vertices if zoomed in to. pseudo-code: IF zoom-in event THEN re-request feature geometry via json and progressively request more detail and vertices. And perhaps with a minimum cap. If it's a simple property with a dozen or two vertices, no need to ever simplify, just request them all once.