Support tested and confirmed this is fixed in Portal for ArcGIS 10.6.
The behavior does not occur when using built in Portal for ArcGIS users at Portal for ArcGIS 10.5.1 or Portal for ArcGIS 10.6.
The behavior does not occur in Portal for ArcGIS 10.6 configured with Integrated Windows Authentication (IWA).
This issue does not occur in ArcGIS Online.
Upgrading to Portal for ArcGIS 10.6 should fix this issue. The users have indicated that they are happy to bring their transition to Portal for ArcGIS 10.6 forward rather than hotfix (see next paragraph).
Updating the source code for specific app templates on Portal for ArcGIS is possible, to use more recent source code for those apps if a general upgrade is not feasible. However for this particular issue no testing has been done to confirm whether updating the source code definitely fixes this particular issue (i.e. the fix may require more than just updated app code).