Converting per capita growth rates for subgroups into contribution to total per capita growth rate
The problem is that you've got two inconsistent definitions of growth rate.

One is driven by per capita debt across the whole population.

The other by per capita debt within each subgroup.

These are inconsistent, because different subgroups have different baseline debts in Q2, and different population sizes. Because both of those things are different, and both are divisors in the calculation of your statistic of interest, then simply re-weighting by population size can't fix the problem.

So you can calculate a meaningful contribution of each to overall growth rate, so that together they satisfy  r1 + r2 +r3 + r4 = r0. But then the individual r1 to r4 won't mean much within their own groups - only in terms of their contribution to the whole. Or you can calculate growth rates for each sub group, as you've done in your question, but then they won't mean much in the overall context.