Attached is an image from wikipedia https://en.wikipedia.org/wiki/Microsoft_Edge and an image from Microsoft https://support.microsoft.com/en-us/microsoft-edge/download-the-new-microsoft-edge-based-on-chromium-0f4a3dd7-55df-60f5-739f-00010dba52cf
Thanks for the pointers here, Oliver. Saba confirmed it was the xhr bug causing the issue and suggested a workaround using a global flag in Edge browser.
Stupendous.
I think I previously negated to mention our wiki entry on the subject: https://github.com/adaptlearning/adapt_framework/wiki/SCORM,-closing-windows,-LMSFinish-and-synchronous-XHR-dismissals