Safari sync across devices causes unwanted behaviour

I have an iMac, MacBook Pro, iPad Pro and iPhone all at the latest software level and signed into the same Apple ID. Therefore I see that Safari tabs are synched across all devices. However, I experience random and unwanted behaviour when accessing websites that require a sign-in. What is that either (1) I get caught in a sign-in loop, namely after signing in to the website, I am signed out again, and the sign in process restarts, or (2) I sign in successfully to a website, but after a few minutes I am logged out without having taken any action (and well before any automatic site log-out).

Is there something I can do to resolve this issue other then turning off syncing?

Thanks

Dietmar