Sulfur cube stores its block in the mainhand slot instead of body armor slot

The bug

In Bedrock, sulfur cubes store their block in the slot.weapon.mainhand slot. In Java, it’s stored in the armor.body slot. This difference prevents commands that otherwise could work across versions from working.

How to reproduce

1. Spawn a sulfur cube

2. /replaceitem entity @e[type=sulfur_cube] slot.armor.body 0 dirt

Expected behavior

The sulfur cube should get dirt inside it

Observed behavior

The command succeeds but has no effect