Definitely encountered this in -o json, not sure if this is also affecting other printer formats

cilium uses iota for TraceToLxc segment, which during decoding gets assumed as default empty value and thus ommitted.

iota:
https://github.com/cilium/cilium/blob/450c79ce5e2fbdeb32833df45c04bd529ff6ff4b/pkg/monitor/api/types.go#L135

parser decode:
https://github.com/cilium/cilium/blob/3d57190d73e0407865452bed93f1d4126e3235fd/pkg/hubble/parser/threefour/parser.go#L430

omitempty:
https://github.com/cilium/cilium/blob/18e5c632fb5013763eedde3aaea9ea9995da7a05/api/v1/flow/flow.pb.go#L1191