Win10 / Edge: need to enable Flash for each session

Unfortunately, there is nothing you can do.  This is a feature in Microsoft Edge, nothing to do with Flash.

Per Microsoft's announcement on The End of an Era - Next Steps for Adobe Flash:

The process will continue in the following phases:

• Through the end of 2017 and into 2018, Microsoft Edge will continue to ask users for permission to run Flash on most sites the first time the site is visited, and will remember the user’s preference on subsequent visits. Internet Explorer will continue to allow Flash with no special permissions required during this time.
• In mid to late 2018, we will update Microsoft Edge to require permission for Flash to be run each session. Internet Explorer will continue to allow Flash for all sites in 2018.
• In mid to late 2019, we will disable Flash by default in both Microsoft Edge and Internet Explorer. Users will be able to re-enable Flash in both browsers. When re-enabled, Microsoft Edge will continue to require approval for Flash on a site-by-site basis.
• By the end of 2020, we will remove the ability to run Adobe Flash in Microsoft Edge and Internet Explorer across all supported versions of Microsoft Windows. Users will no longer have any ability to enable or run Flash.

All major browser vendors are moving in this direction as Flash Player's end-of-life approaches at the end of 2020.