UpdateExisting in asset update breaks the asset?

[TheCypherGoat]

Describe the bug
When the 'updateExisitng' box is used in asset update, the asset can no longer be viewed in asset (you get server 500 error)

To Reproduce
Connect a trigger to an asset update, fill in details of an asset, select 'updateExisting', run trigger

Expected behaviour
Any fields specified are added to the asset

Screenshots
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Environment (please complete the following information):

Additional context
This seems to be because the updateExisitng fails to put the 'update source: manual' field in the data. Replacing this via the model editor returns asset to working state (though other data was also copied).