If you upgrade a viewer, it will compare the contents of the existing viewer with the new viewer to determine what should be replaced and what shouldn't. If you have modified the index.html (for example, to shorten the launch URLs) then you may decide to keep the existing Index.html file rather than using the new one. Certain releases change the libraries used that are referenced in the Index.html file and keeping the older Index.html file could cause issues with the viewer. Without these libraries, we will get strange behavior - for example, unable to see feature attributes due to an error only visible in the browser console:
Uncaught TypeError: Cannot read property 'ChartPointCollectionAggregator' of undefined.
This is just one example, the results will be unpredictable.
To resolve this issue, use WinMerge or a similar tool to merge the necessary changes into the Index.html of the viewer.