Following the documentation it seems like

!literal

should be working here:

field_template: !literal |
  {%- if alertDef.entityLabels.channel is defined -%}
    {{ alertDef.entityLabels.channel }}
  {%- else -%}
    _sandbox
  {%- endif -%}

but I get error

*Error:* template: templates-all-atmos-sections:624: function "alertDef" not defined

, is there something wrong with my usage or a bug? I tried with

1.210.0

&

1.216.0

LGTM, seems like a bug.