The logic varies based on your feature geometries and if there are unique value fields in your target layer (the layer you are copying the selected feature-s into).
If your target layer has a field that only allows for unique values, then you will only be able to create one feature at the time.
- In the case of a target point layer, since multi-part features cannot be created, you will be prompted to select only one feature. Multi-part features are only allowed for line and polygon layers. This is a known limitation for the copy capability.
For example below, the target is a points layer with a unique values field:
- In the case of a target line or polygon layer (in your case above, it is a line layer - so this is your case), you will be prompted to create one feature or a multi-part feature from multiple features selected. In this case, only the "Create multi-part feature" button is shown if multiple features are selected (multiple checkboxes checked), and if 1 checkbox is checked then the "Create feature" button is shown.
For example below the target is line layer with a unique values field:
Here's an app with some example cases you can play around with: https://www.arcgis.com/apps/webappviewer/index.html?id=d719bd42e2864cb09d3e7add47e6bac2
Now, if there are no fields in your target layer that only allow for Unique values (excluding the system fields), then what you are seeing could be related to the issue I've mentioned in the comment above and then using the Smart Editor widget from the latest version of the Web AppBuilder Developer Edition 2.23 as a custom widget in your Portal would be a good workaround.