Sound through the earphone is also sent to the amplifier when using a jack splitter after a audio mixer
A dynamic microphone consists of a permanent magnet, and a coil of wire attached to a diaphragm.  If you speak into the diaphragm, it generates a small current in the coil.

A headphone speaker consists of a permanent magnet, and a coil of wire attached to a diaphragm.  If you pass an alternating current through the coil, it vibrates the diaphragm and makes a sound.

So a dynamic microphone and a small loudspeaker are actually the same thing.  This means that you have effectively connected a pair of microphones to the input of your amplifier.

As others have said in their answers, the solution is a small headphone amplifier between the mixer and the headphones.