Issues logging into apps on new device

Sorry if I've missed this. Veteran Apple user. New iPhone15+ - same iOS (17.5.1) as my iPhone SE Plus. Same apps.

But, with both Wyze and Slack - I am unable to log in using my existing (still working!) username/passwords. In both cases, I receive an error message that I have the wrong username and/or pw. BUT, I'm able to sign in on other devices.

I've cleared website data in Settings. I've fiddled with cookie settings. What am I missed? (I've done every trick I could find online and from the app support folks)