@Aran Billen, Thanks for posting in Q&A. From your description, I know we have configured "Max inactivity time device lock". And the registry key is applied. But the device is not lock after 30 minutes. but closer to 10 minutes. If there's any misunderstanding, feel free to let us know.
Based as I know, if one GPO with the same setting applied to this device as well, the GPO will take effect. Therefore, please check if any GPO with the similar setting applied to this device. If yes, remove it to see if the result will be different.
Hope the above information can help. If there's any update, feel free to let us know.
---If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.