@Martin, Thanks for posting in Q&A. From your description, I know the device password already contain 6 digits. But it is failed. Based on my research,
On Android 12 and later, if the Required password type setting is changed from the Device default value in a policy, then Newly enrolled Android Enterprise 12+ devices will automatically use the Password complexity setting with the High complexity.
Here, we suggest create a new policy for Android Enterprise 12+ devices and configure the Password complexity setting to see if the result will be different.
If there's any update, feel free to let us know.
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