Oh you’re using the once command? You should have mentioned this and all other details at the the start. Otherwise I’m assuming you’re following the standard offline update process.
The once command doesn’t work with Lockboxes on more recent versions of Torizon OS. What probably happened is that when you built your custom image with Yocto, it ended up using a more recent version of Aktualizr where once no longer works with Lockboxes.
When you used the pre-built images I assume you’re using an older version of Torizon OS where once still works with Lockboxes. If you want to do this in Yocto, then you need to pin Aktualizr to a version where this still works like this other user here did: Offline updates not searching for lockbox - #10 by Mowlwurf
Best Regards,
Jeremias