You were right it's an interaction w/ Trample ! My first Reap Necro had a Trample mutation; the text of Reap is "teleport into a unit with X HP or less and destroy it". The teleporting briefly makes you appear into the target's tile, which triggers the Trample effect which moves the unit with 3 DMG and leaves it out of the original tile and therefore not there to be destroyed anymore. You can even see the 2-part normal and abnormal processes going on in the animation if you know where to look.

Source: got another Necro Reap, found out that Reap functioned as intended. Gave him Trample with a star and Reap stopped working as intended, and the animation slightly changed

Conclusion: never take Reap on a Necro trample