1.Try to configure the app permissions of Duolingo to permit InApp opening.
2. Try to use an email app that allows you to select where to open the password reset link (i use FairEmail, second screenshot). Maybe that’s the trick?
The following screenshots are in German, but it should be understandable:
Duolingo app details - permissions:
FairEmail password reset link opening
Tested it just now: Duolingo apps pops up 