Hi Ben,
What release of Portal for ArcGIS are you using? As Robert Scheitlin, GISP commented, typically the version of Web AppBuilder included with Portal is slightly behind in functionality compared to Web AppBuilder in ArcGIS Online.
FYI, Portal for ArcGIS 10.5.1 includes Web AppBuilder v2.4.
I suggest that you download and use Web AppBuilder for ArcGIS Developer Edition with your Portal instance. So you can leverage the lastst functionality with your GIS assets.
Hope this helps,