Dell desktops aren't getting DHCP addresses after reboot. Where should I look?

Start with a packet capture on an affected desktop during boot, plus the DHCP server logs or a capture on the server. Follow the DHCP Discover, Offer, Request and Acknowledgment exchange and see where it stops.

If the server is on another subnet, check the relay or helper configuration along the path too. A manual renew working doesn't tell you whether the initial request was sent, reached the server, or got a reply back.

Check the scope for available addresses and look for an active lease for a machine that failed to get one. A reservation is worth testing on one affected PC, but compare the result with a normal lease. It won't fix a request that never reaches the server.

The reboot pattern makes the wired link and adapter drivers worth checking. Compare driver versions and settings on affected and unaffected desktops, and check whether the switch port is ready to forward traffic when Windows starts DHCP.

Review the switch's spanning tree configuration for client-facing ports, including its edge or portfast equivalent. Don't change that blindly on ports connected to other switches.

I wouldn't pin this on KB5101650 yet. Removing it from one machine didn't resolve the issue, and the symptoms alone don't establish a cause. Compare update and driver versions across working and failing clients, then test changes on a machine you can reach locally.

Be careful with scheduled release and renew tasks during remote troubleshooting. If the machine doesn't come back on the network, you may lose access before you can inspect it.