There is an annoying problem with the e-mail app and QR codes.
I have turned dark mode on.
So, e-mails backgrounds are dark which is stopping QR codes from working in e-mails.
When disabling dark mode and restarting the phone, the e-mail app stays in dark mode.
I cannot reproduce this in that dark and light theme switch on and off in Mail app with the rest of the system, perhaps you could share your device details like this