I am also having this issue. In perspective I can create a SESSION CUSTOM property like I can on any other view, but I cannot save this change. Saving shows ‘No changes to save’ in the bottom left corner and closing and opening the designer wipes any custom session property I added.
The workaround, restarting the ignition server, did not work for me as of the latest build `8.0.0-beta0 (b2019032002)