One thing I thought of related to your question that you didn't mention is enabling and configuring the popups once a map service is published. If you have published and set up online maps in the past, you have done this. If others have performed the publishing/setup operations, you might not be aware of this. This has to occur in the desktop browser application, using either the traditional Map Viewer or using Field Maps Designer.
After a map is published, whether they are editable layers or non-editable, The pop ups need to be first enabled, then configured. Configuring allows the map author to choose which fields to display in the pop-up, the display order, how many decimal places to show, etc. Once done, the config changes will hold until reconfigured. Older version of the online mapping application would "forget" these config changes when a map was republished. Newer versions of the software have gotten better in that regard, but checking your pop ups is always merited following republishing.
If your user role allows you to author/edit online maps, you enable/config the pop-ups by clicking the ellipsis (...) to the right of the data layer in the Table of Contents on the left side of the online map. If you are using a Web App in the field, you have to make the pop-up changes in the parent Web Map, which will then carry over to the Web App.

When you choose "Configure Pop up" a dialogue box opens that allows you to define how the popup will appear to the end users when a feature is selected. A second dialogue box is available from that screen for further configuration, allowing the author to choose which fields are visible and which are editable, giving the map author good control of the online map experience.

OK your changes on both screens to get them saved and back to your online map where you can test out your changes. Keep in mind that end user map caches may need to be cleared, in order to see the new changes.
Hope that is useful. Maybe you already knew all of that and you still have another issue.