When McDonald's switched from email/password to magic links I had a hard time getting the magic link to work with the McD app. It usually would just open in the McD website.
Thus was quite annoying because about 98% of the time I eat McD's I would not do so if I could not order via the app [1].
I finally gave up and switched to using "Sign in With Apple" (SIWA). There was no way that I could find to add SIWN to an existing McD account, so had to use the SIWA that hides the real email from McD. That created a new McD account so I lost the reward points that were on the old account, but at least I could again use the McD app.
[1] They have a weekly "Free Medium Fries on Friday" deal in the app available for use on orders of at least $1. Almost every Friday for lunch I make a sandwich at home and then get cookies and the free fries to go with it from McD.