When updating via rpm, rpm managed files should be removed or added as expected. It is possible you are hitting an error like the one described here: https://github.com/OSC/ondemand-packaging/issues/42.
If you ever installed OnDemand pre-rpm, or manually build the dashboard app assets, it is possible a .sprockets-manifest file that generated that is not rpm managed and thus is not removed on upgrade.
Do you by any chance see multiple .sprockets-manifest files in /var/www/ood/apps/sys/dashboard/public/assets?
I believe the manifest file you want to keep in v1.8 is /var/www/ood/apps/sys/dashboard/public/assets/.sprockets-manifest-c47593093860eba73c09e1d0ed80d2d5.json