Cannot delete a tag added through an Asset rule

Hello,

We had created in the past an asset rule to assign a tag to a few machines. Now we are trying to remove the tag but we can't find the right way.
We have delete the Asset rule. (it was turned off more than 2 months ago)
When I go to the machine details and click on 'Manage tags', I can see a section called 'Manual tags' (there I can add remove tags from the console) and a section called 'Rule-based tags' with the description 'Rule-based tags are automatically added to devices based on rules that you create. You can add, edit or delete a rule in Manage rules.'

Going through powershell and the API, it doesn't work either. Even getting the details from a machine only shows the manual tags.

How do we remove then such a tag ?

Thanks in advance for your help.

Marc