The Symantec Endpoint Protection (SEP) for Mac agent has made use of a Network Content Filter (NCF) that installs with every agent since the new Mac agent design released with 14.3 RU1. The NCF interacts with Apple's Filter Data Provider which is otherwise known as the network stack. The Mac agent's Firewall and Intrusion Prevention System features rely on the NCF in order to properly evaluate the traffic in the network stack.
When troubleshooting network interruptions where you suspect that the SEP client is contributing to the issue, please first take a look at the below document which will assist in isolating the issue to either a feature, policy or possibly the NCF component itself.
How to troubleshoot SEP / SES for Mac - disabling network filtering and other components
If you have isolated the issue down to strictly whether or not the NCF component is enabled or disabled, then often the only workaround is to keep it disabled until the product team can identify and release a fix for the issue.
Endpoint Protection for Mac 14.3 RU1 and newer
macOS 10.15 and newer.
The best thing to do at this point is to open a case with support and be prepared to collect network related data while reproducing the issue.
Here is a list of data that support may need to move forward.
Make sure to compress the data and upload it to your support case.
How to Debug Symantec Endpoint Protection SymDaemon on the Macintosh client
Gathering information about Symantec products on a Mac using GatherSymantecInfo
How to generate star platform debug logging for Endpoint Protection Mac client