Hello Elizabeth,
It sounds like you've chosen to author your Event Editor webmap using a local file on the web server where the app is deployed. May I ask what version of Roads and Highways you are using? It also sounds like you have portal running so I might also ask whether there is any specific business reason to use the local webmap as opposed to simply authoring your webmaps on portal?
Outside of those questions, the LRS service you are using for webmap configuration usually defaults to the zoom scale the MXD you published from was set to. You might want to check on that MXD. Additionally, do you have any feature layers in your map without a spatial extent set? Sometime empty event layers can cause this if you set the initial extent to the full map extent:
You've likely already found our resource document for this topic but I'm adding it to this thread just in case. Authoring Webmaps for Event Editor
Incidentally, extent (among other things) in Event Editor can also be controlled via URL parameters. https://enterprise.arcgis.com/en/portal/latest/use/use-url-parameters.htm
And finally, our resource document on smart launching Event Editor through URL parameters: https://enterprise.arcgis.com/en/roads-highways/latest/event-editor/smart-launching-event-editor-thr...
Hope that gives you a good starting point to resolve your extent issue.
-amit@esri
Esri Transportation LRS Team