For tier 1 troops.
Sending troops to rear gives:
2 bronze at level 3
5 bronze at level 4
28 bronze at level 5
Cost of upgrading from 3 to 4 is 6, from 4 to 5 is 18.
After upgrading:
28 - 18 = 10, for level 4
28 - 18 - 6 = 4, for level 3
So upgrading level 3 and 4 troops to level 5, doubles the bronze You receive when sending them to rear.
Enjoy while it’s not fixed