Both Microsoft Editor and the new spell checking engine rolled out very close to each other.
Microsoft Editor is an AI-based style helper that goes beyond orthographic and grammar correction.
Both correction engines compete with each other and result in inconsistent behavior.
This, I think should have greater collaboration between Edge and Microsoft Editor Teams.
I have tested scenarios using only only one or both active at the same time, and when both are active, sometimes settings will not "stick" .
Within OWA for example, when both Microsoft Ediotor and the browser native spell checker are active, I have seen cases of double red squiggles, and red squiggles highliting all words as not recognized, even with the same language selected on both tools.
As a user, it becomes erratic and confusing. I could not pinpoint where the bug actually is just by trying to change the swttings in succession. Maybe both tools are reading and writing the browser's settings, leading to an inconsistent behavior.
Bottom line, I think the Edge team should get in touch with the Microsoft Editor devs and collaborate to weed out this inconsistency.