When net-mgmt/net-snmp is compiled without ipv6, net-mgmt/php74-snmp then fails with
fatal error: 'net-snmp/library/snmpUDPIPv6Domain.h' file not found
do you have some buildlogs for this?
Most important: have you disabled IPv6 in lang/php74 before compiling the snmp module?
There is no dedicated way to disable IPv6 in snmp. I am guessing that this issue can vanish when disabling IPv6 completely when compiling lang/php74.