localePolicy.cultureName of numerical tabs is replaced by en-US when the tab is placed with anchorString (kept original with fixed position)

Hello everyone,
when I create an envelope via POST /v2.1/accounts/{accountId}/envelopes with inline tabs, the localePolicy of my numericalTabs is not kept if the tab is placed by anchorString. 
DocuSign changes the cultureName to "en-US" and to en-US currency formats. If I place the same tab with documentId/pageNumber/xPosition/yPosition, localePolicy.cultureName stays "de".

What I send (anchorString, minimal):

{
  "status": "sent",
  "emailSubject": "Test",
  "enableWetSign": "false",
  "documents": [ { "documentId": "1", "name": "test.pdf", "fileExtension": "pdf", "documentBase64": "[...]" } ],
  "recipients": { "signers": [ {
    "recipientId": "1", "routingOrder": "1", "name": "[name]", "email": "[email]",
    "tabs": { "numericalTabs": [ {
      "tabLabel": "n1",
      "anchorString": "AMOUNT",
      "validationType": "number",
      "localePolicy": {
        "cultureName": "de",
        "currencyPositiveFormat": "csym_1_period_234_period_567_comma_89",
        "currencyNegativeFormat": "minus_csym_1_period_234_period_567_comma_89"
      }
    } ] }
  } ] }
}

What GET …/recipients/{id}/tabs returns:

"localePolicy": {
  "cultureName": "en-US",
  "currencyPositiveFormat": "csym_1_comma_234_comma_567_period_89",
  "currencyNegativeFormat": "opar_csym_1_comma_234_comma_567_period_89_cpar"
},
"anchorString": "AMOUNT",
"anchorUnits": "pixels",
"anchorTabProcessorVersion": "v1_3",
"xPosition": "[...]", "yPosition": "[...]"

Same request, but with fixed position instead of the anchor returns:

"localePolicy": {
  "cultureName": "de",
  "currencyPositiveFormat": "csym_1_period_234_period_567_comma_89",
  "currencyNegativeFormat": "minus_csym_1_period_234_period_567_comma_89"
}

More observations:

• Same for formula tabs
• Number tabs created in the web UI in templates keep “de”, and the signing view shows 1.234,56.
• The user settings (localePolicy.cultureName: "de") and the account setting are German.
• The result is the same when I leave out anchorUnits, offsets, minNumericalValue/maxNumericalValue, and emailNotification.
• Same client and same HTTP headers for both requests.

Questions:

1. Is this known? Does the anchor processing (anchorTabProcessorVersion: v1_3) transform tabs and drop localePolicy?
2. Is there a way to keep the locale with anchored tabs, or to set it after anchor placement (for example PUT …/recipients/{id}/tabs on a draft)?

Thanks