Welcome to Microsoft Q&A Platform, thanks for posting your query here.
This behavior is usually triggered by a GPO that applies a session timeout setting or a screen saver setting that locks the session after certain minutes.
You can check the GPOs and see where these configurations are done.
Once you have modified the group policy setting, it may take some time for the changes to take effect on the AVD host pool. You can also force the policy to be applied immediately by running the "gpupdate" command on the AVD host pool.
Hope this helps.
If you need further help on this, tag me in a comment.
If the suggested response helped you resolve your issue, please 'Accept as answer', so that it can help others in the community looking for help on similar topics.