When active, the multiblock hologram make the player's arm transparent, disappears while sneaking, and moves with the player's point of view during the first person sneaking animation.
Versions:
Fabric API - 0.3.0+build.207
Reborn Core - 4.0.0.80
Tech Reborn - 3.0.0.78

Fixed, will be in the next RebornCore version.