Yes, Servicing Stack update can’t be uninstalled.
You could restore system to a previous time that system didn’t install this SSU.
Or contact HP support for help, maybe they can give you a updated firmware which is compatible with the latest Windows 10 SSU.
Another method, degrade system to 20H2 from 21H1, after install HP Firmware 15.18.0.0(2) complete, upgrade to 21H1 again.
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