This feels like an integer overflow. Maybe the variable “last” got too big

It can be overflow.

from math import log2, ceil
 
for test in range(int(input())):
    n = int(input())
    a = [int(x) for x in input().split()]
 
    res = 0
    last = a[0]
    for i in range(1, n):
        c = a[i]
        p = 0
        if c < last:
            p = ceil(log2(last / c))
            res += p
        last = c * (2 ** p) //This line
 
    print(res)

In marked line you are multipling c by number that can be really big(There is some if-s to fix this).