I have the same issue with Antimalware Service Executable on Windows 10. ThrottleStop 9.3 is the last version that properly throttles the MS Defender as well.I am not sure if there are two separate issues or only one issue.
For some users, running TS 9.3 can instantly fix the less than 100% C0% problem. TS 9.4 and newer makes fewer automatic changes to fix throttling problems. I have no idea why TS 9.3 has this magic ability while newer versions do not, even when both versions are setup exactly the same. Without hardware with this unusual throttling issue, there is no way for me to find out what is causing it.
If you need some logs, I can provide them.
9.3 vs 9.3.1