Final update: We would like to inform you that this issue is fixed now. This issue occurred due to an UX update we did for the lab VM create flow in Labs. Users should now be able to edit the base image while creating a formula. Please let us know if you are still experiencing any issue while using this functionality.
We thank you for your patience.
Update: We would like to inform you that user may not be able to edit the base image while creating a VM from a formula or while editing a formula. This is due to a bug that the team is actively working on fixing. We will update this post as soon as we have deployed a fix.
We apologize for the inconvenience.