Hello Vedran,
After having a discussion on the above issue and tried many resolving techniques, glad that the issue is resolved as expected using Azure CLI method. I am summarizing the steps below to create or configure alert processing rules properly without any conflict. Using az monitor alert-processing-rule create CLI command resolved the issue in your scenario and the detailed command has given below.
az monitor alert-processing-rule create --name 'AddActionGroupTosub' \
--rule-type AddActionGroups \
--scopes "/subscriptions/subscriptionID" \
--action-groups "/subscriptions/subscriptionID/resourceGroups/resourceGroupName/providers/Microsoft.Insights/actiongroups/ActionGroupName" \
--resource-group LogAnalyticsDefaultResources \
--description "xxxxx"
I have tried the same execution in my environment and was able to create an alert processing rule successfully.
Hope this helps!
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