Historically, installing Symantec Endpoint Protection (SEP) disabled Windows Defender on Microsoft Windows operating systems.

However. starting with Windows 10 1903, Microsoft required AntiMalware products wishing to integrate with the Windows Security Center (WSC) must do so through an AM-PPL process.

SEP/SES does this via the Symantec Endpoint Protection WSC Service service (SepWscSVC.exe), which tells Windows that SEP/SES is registering as an AntiMalware product and will stop Windows Defender AntiMalware service.

However depending on how Windows Defender has been configured via group policy, SEP/SES may not be able to stop Windows Defender AntiMalware service

Review group policy settings for Windows Defender.

For more information please see the following Microsoft link and/or contact Microsoft.

Microsoft Defender Antivirus compatibility with other security products