pfSense

If you disable an interface that's a VLAN, it deletes the VLAN, and re-enabling that interface doesn't add the VLAN interface back.

For example, to replicate:
Interfaces>assign, VLANs, add a VLAN. Back to assignments tab, assign it as OPT1.
Go to Interfaces>OPT1, enable the interface, put a static IPv4 IP on it, save and apply.
Go back to Interfaces>OPT1 and disable the interface. Note the VLAN interface is deleted at this point.
Re-enable OPT1 and the VLAN interface will still be gone.

When disabling a VLAN interface, it shouldn't ever delete the underlying VLAN interface, it should only remove the IP config from it. VLAN interfaces should only be deleted when they're deleted under Interfaces>assign, VLANs tab.