Back at the beginning of 2018, I submitted a github issue about new-user.php not being automatically patched in WP 4.9+: https://github.com/wpsharks/s2member/issues/1132
This issue has been closed a long time, and supposedly has been patched in a dev version shortly afterwards, however, the latest downloadable version is still from 2017 and it does not include this patch.
This issue represents a compatibility issue with Multisite installs, and it would be great to see whatever patch was put in place for this over a year ago to ultimately be released for us who depend on this feature and cringe whenever a WP update is released.
Thanks for looking into this and for bringing us an updated Pro version!
~Cam