The sign-in can be working correctly
ZDF documents using an existing ARD account to access personalised ZDF features without creating another ZDF account. The same explanation says the services exchange an anonymous user identifier and keep other settings and viewing data separate. A successful login followed by an empty ZDF watchlist can therefore be consistent with the published design rather than proof that your ARD list was deleted.
Look in the service where the item was saved
If you saved a programme in ARD, return to that service to check its watchlist. If the programme is also available through ZDF, you can locate the actual ZDF item and save it there separately. Match the episode and edition rather than assuming two programme pages with similar artwork represent one shared record. The login does not make every programme available in both catalogues.
Keep age access and password management separate
ZDF’s shared-login explanation identifies age verification as a function requiring a ZDF account and says an ARD password is managed through ARD’s own account controls. An authentication link is not an instruction to change both services’ settings in one place. Use the correct publisher’s current instructions for the particular account action you need.
Describe a missing-list question accurately
State which service originally held the item, which login you used and which list you are now viewing. Avoid repeated account creation as a test. This article applies the services’ published separation and does not inspect an account, migrate personal data or recover a deleted entry. It also does not establish that a related broadcast is included in a UKIPTV subscription.
Questions, answered
Does ARD sign-in merge my watchlists across both services?
No. ZDF explicitly says settings, lists and watched videos remain separate.
Should I reset my password because my ZDF list is empty after ARD login?
First check where the list was created. Separate lists can be the expected behaviour.