That is because “Legends of Today” is actually not an episode of Arrow. It’s one of a few crossovers with The Flash.
Hence you will only find it listed in the metadata sources under The Flash: The Flash (2014) - Legends of Today (1) - TheTVDB.com
Arrow has also crossovers with Supergirl and Legends of Tomorrow, btw. So you are likely to encounter similar issues with these episodes.
If you also have these 3 other shows on your server, you may want to look at Watch crossover episodes of several series in-order
