I'm using ansible-playbook 2.7.7.

The nosoftlockup configuration in host_vars:
grub:
nosoftlockup: True

Results in the string "nosoftlockup=True" being configured on the kernel command line, but we only want to configure "nosoftlockup".

Setting it to null (nosoftlockup: null) fixes this, but raises another issue when checking the command line:
failed: [10.30.51.69] (item=nosoftlockup) => {
"assertion": "item in ansible_cmdline and grub[item] == ansible_cmdline[item]",
"changed": false,
"evaluated_to": false,
"item": "nosoftlockup",
"msg": "Kernel parameters! != True"
}

The " != True" is a debug I added. It seems that grub[item] is an empty string (with nosoftlockup: null), but ansible_cmdline[item] is True when only "nosoftlockup" is configured.