Purview Assign Label using GraphAPI premium

Hello !

I'm trying to use the following premium GraphApi call (assignsensitivitylabel)
to apply sensitivity labels to files on OneDrive and Sharepoint files. My application is correctly configured to use this premium/metered API (valid subscription, appropriate API permissions, no spending limits on billing account, etc) and I can call the API as documented. The problem is that, whatever file I try to label (docx, xlsx, pdf, pptx), I always encounter this error:

{
  "error": {
    "code": "notSupported",
    "message": "Sensitivity label cannot be assigned due to unsupported feature.",
    "innerError": {
      "code": "unprocessableEntity",
      "date": "2025-04-30T14:18:37",
      "request-id": "4424a42b-6253-4450-a0de-6344e722a16c",
      "client-request-id": "4424a42b-6253-4450-a0de-6344e722a16c"
    }
  }
}

For the sake of testing, I tried labeling a txt file and I got the expected error about the fact
that txt file is not a supported format so it makes me think that it really is trying to perform the labeling operation. If I download the file locally on my machine and perform the labeling operation using the Purview SDK (latest 1.16.149 version), it works as expected and the sensitivity label is applied so it's definitely not a problem with the files themselves. Can you give me some hints on what could prevent me to use the Graph API assignsensitivitylabel operation?
I looked like everywhere, ask GPTs, did and redid all the required steps to enable that API but with the same results all the time.

Thank you for your help!!