how does your boot_command looks like? mine is working like this with Rhel8.2:
"boot_command": [
"<up><wait><tab><wait> text ks=http://{{ .HTTPIP }}:{{ .HTTPPort }}/ks-manual.cfg<enter><wait5>"
],
"http_directory": "kickstart",
"http_port_min": "8000",
"http_port_max": "8000",