I had an issue with apt update when IPS is enabled. Try disabling IPS temporarily (this is just a test).
For me there was single rule I had enabled that caused the problem. (Sorry I cannot remember the rule but I’ll keep looking for my notes!)
EDIT: Found it!
In an IPS rule named: emerging-policy.rules
Is a item called:
ET POLICY GNU/Linux APT User-Agent Outbound likely related to package management
And it was blocking my apt-get update.