i found another solution that you change configuration in $SPLUNK_HOME/etc/apps/alert_manager/default/collections.conf , then add text below in bottom of the files
[drilldown_settings]
enforceTypes = true
field.type = string
field.field = string
field.disabled = bool
field.label = string
field.search = string
field.comment = string
save its and restart splunk service, after that the WARNING is nothing 😄
reference change from github:
https://github.com/alertmanager/alert_manager/commit/d6b0836579805428217dac2c5b0a5a0e32b4c87f#diff-26d29611bf210eea85b03369f1c811d0
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