@dmeyer thanks for the tip. However, this would involve several requests from the rest “side”
scenario 1:
1 create a property
2 send the new project data
3 jql query to check if there is this property
4 delete this property : (a) this has to be done in case a jira UI update occurs and (b) when do you decide to delete it ? - after all you’re not getting the webhook event now, so this becomes a race condition
scenario 2:
1 create a property
2 send the new project data
3 receive webhook
4 api check for existence of said property
5 delete property
scenario 3:
1 delete a property
2 send the new project data
3 jql check for non-existence of property
4 api create this property : (a) this has to be done in case a jira UI update occurs and (b) when do you decide to create it ? - after all you’re not getting the webhook event now, so this becomes a race condition
As you can see, all very messy. If only we had a “source” header (“jira/rest/api”) 
Also, (Project webhook changelog) could you tell me why the project webhook does not receive a changelog ?