Nice looking tank! Seems like things are doing well for 4 months in. As nrc mentioned, a cleanup crew can make short work of a small fish if it happened to expire from other causes. Also, those are both “perches” so could have run afoul of your large bubble tip.
In general, I would hesitate to add a mandarin to a relatively new tank as they are normall hesitant to take prepared food and rely on an indigenous pod population. If you weren’t supplementing with live pods (or have a well established refugium) it may well have starved 🙁 If you decide to replace him, try to find one already adapted to frozen food - you might have better luck.