@arjen05 has a valid point here:
Your original flight is BRU-AMS-USA. With the current entry requirements, you will probably be required to check in for the flight at a physical check-in desk at your original departure point, which is BRU, where they will verify your covid documentation and perform the required extra security screening (questions) that airlines are required to do for US destined flights. Check-in for KLM intra-EU flights closes 40 minutes prior to departure, so you won't be able to get off, go to the check-in desk and return to the flight at BRU.
You might be lucky and be able to check-in online, or you might be able to get everything done in AMS, prior to departure to BRU. However, both are not something that I would count on. Is there no other option to position yourself to BRU? An earlier flight? Train? Have someone drop you off in BRU?