I kept reporting this issue but they kept rejecting it 
Basically when using Final Gather mode, the indirect bounces setting is unfrozen but the indirect bounces value has no effect. It has only effect on Brute Force GI mode, which is much slower but allows you to use multiple indirect bounces to get closer to path traced result.
I tried to explain it to the Bug Report submission staff twice but they never understood it. Unfortunately, sometimes when you report UE4 bug, your bug may be triaged by someone who has little to no understanding about the area of the engine the bug occurs in, and will just haphazardly reject it 