This seems to have to do with a firmware update that just came out as I never saw this issue before this morning, and your post is exactly from today! I went through the same steps that you have as well. I caught a phone rebooting when I came in this morning and it must have been due to an update.
For the time being however I added the following to our provision template on our T46U phones. According to Yealink documentation, this will not work on T48G, T46G, T42G. T41P, and T29G phones.
# Determines whether the phone will display warnings. These include default password, account
# regisistration failed, provisioning credentials wrong, network unavailable, and line on redundancy
# registration. Default setting is 1, but this is set to 0 to prevent a false positive warning of
# default password being in use.
phone_setting.warnings_display.mode = 0 The source of where I found that setting is the admin guide on the Yealink phones found here. I would've attached it but the file size is too large! The comment block is what I typed up for future reference.
To add a provisioning template you have to go in to Phone System Management -> Company Info -> Account Settings -> Provisioning Template in the Desk Phone section. Once you create a provisioning template, you have to go to your company's phone device listing and actually apply the provisioning template by checking the boxes of all the phones you want to apply it to and selecting Bind
