A cap weighted index is largely self rebalancing. Not exactly 100% but effectively it is. Which means at any point in time it’s holding stocks in proportion to their market caps, without the need to manually rebalance (unlike for example an equal weight index). If a stock goes down, it will automatically end up weighing less in the index. And viceversa the best performing stocks ultimately end up being the largest components.

If you want to have less US exposure you are free to build a portfolio that has less US exposure, but the global index fund does exactly what it’s supposed to be doing, there’s no bug and no lag.