Hi Julien, thanks for reaching out!
Target ROAS is currently supported in bidding strategy reports as a dimension at account and campaign level only, If you are using this at Ad group level in the Google Ads UI could you provide some detail to show where you are using this (screenshots would be useful). We can then review.
The alternative is the Average Target ROAS metric. This is available through the Google Ads API, and we have added this to our product backlog for future consideration, but are unable to give any commitments or timelines of when we may be able to add this.
Thanks,