AWS load balancer health check getting 400 on Windows 2022

Question

I've just set up a new ALB in AWS. It's using http2 and the health check is trying to load /lb/lb.htm from the web servers via https on port 9001.

In Windows port 9001 is open in the Windows firewall.

In IIS I have an https binding on the default site on port 9001 with no host header. This page loads successfully from a web browser on my laptop, but when the load balancer calls it, it gets a 400 response, and I see 400 "Bad request" entries in httperr.log. It does log that the request http/2, there is no host header, and it is requesting /lb/lb.htm. No other errors seem to be logged. The binding does not have http2 disabled. I've also tried disabling TLS1.3 and legacy TLS for the binding but that made no difference. I'm not seeing TLS errors in the event log.

This set up worked with Windows 2019, so does anyone know if something changed in 2022 that could cause this? Alternatively, is there any way to get a more detailed error than "bad request"?