Choose the kind of memory you need
If your goal is an alert for tonight’s episode, use the dated reminder. If you want a programme kept in a longer-lived list, use the favourite. A favourite alone should not be treated as evidence that an alert for a particular broadcast has been scheduled.
Consider a weekly series: keeping the series handy and receiving a reminder for one dated episode are separate needs. Check each control that you actually used. A reminder disappearing after the programme is consistent with Freeview’s described lifecycle, rather than automatically indicating that the favourite was lost.
Finish the first reminder setup
Freeview documents reminder controls in the TV Guide programme pop-up and the programme’s More Info page. On first use, its instructions have you configure reminder preferences and then return to the programme to select the clock again. Setting the preference is not the final programme-selection step.
After selecting the intended event, inspect the Reminders list under More and confirm its date and time. That visible entry is more useful than relying on the memory of having visited the preference screen. Check that the selected event is the desired episode or showing rather than a repeat with the same series name.
Review the correct list afterwards
Freeview places Favourites under More as well, with saved items presented alphabetically in its documented interface. If looking for a long-term selection, inspect that collection instead of expecting a past reminder to remain among future alerts.
For a missing future reminder, record the programme, broadcast date, guide region and whether it appeared in the Reminders list immediately after selection. For a missing favourite, note whether app data was cleared or the heart was removed. Keeping these two histories separate makes the question specific enough to investigate without assuming one icon performs both jobs.