Thank you all for your patience!
I’ve worked closely with our support and dev team to investigate this behavior and I can confirm this is not a bug or the result of a recent change; this is in fact how custom fields in multi-homed tasks have behaved since their original launch; but let me give you more context to understand why this is happening:
In our guide article dedicated to custom fields, we mention that “Anyone with access to a task can modify the values of its custom fields.” This is because the data from custom fields “lives on the task”, so even if you don’t have access to the container project that bestowed the field definition on the task, you can still see any data set on the task.
In this specific case, you’re correct, Jandieg can indeed clear the data for the Estimation field. This does not remove the field from the Budget project, but does clear the field value for all users. And I see why this would be a concern; although it hasn’t been prioritized yet our team is planning to improve this workflow in the future; when they do, I’ll make sure to post in #community-forum-announcements or keep you posted on this thread. In the meantime, if someone clears one if this fields, note that they do appear in your task activity.
Again, we do recognize that there is room for improvement here and rest ensured that our team is aware of it and will look into a better solution for this; so stay tuned!