Read the series name before the number
Several shows share names. The Diplomat here is the 2023 series with Keri Russell, not the other 2023 production with the same title. Cast and first-air date make a quick identity check.
Why the list and a service can differ
Season records describe a production's aired episodes. Viewing services can offer different seasons in different countries. A newly aired episode is not automatically present in every edition on the same day.
Save a useful checkpoint
Choose the season and last completed episode on the series page, then save. This is a reminder you control. It does not watch your player or infer that you finished an episode because you opened its page.
If one episode will not play
Compare the title and number in the viewing service's own listing. Try another episode there to distinguish a local episode fault from a wider player problem. Keep your checkpoint at the last episode you completed.