The ArcGIS Maps SDK for JavaScript 4.26 MapImageLayer pop-up fails to open when a custom text pop-up is configured and not all features in the layer are symbolized.
This issue is fixed in ArcGIS Maps SDK for JavaScript 4.27. Refer to https://github.com/Esri/feedback-js-api-next/blob/main/CHANGELOG.md#bug-fixes-and-enhancements for an outline in the 'Change log' section. Refer to https://js.arcgis.com/next to test the fix using the next CDN.