In reply to rag123:
The issue is with the iff expression
Based on the code the expression is True ONLY when (bit_idx == aidx) is 1'b1
Bins are hit only when the iff expression is True which occurs during your 2nd iteration of for loop when i == 1
For all other iterations the expression is False and no bins are hit
If you comment the iff expression you observe 100% coverage