Field Layout breaks when '#tree' => TRUE is used in an entity form

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

Not sure I follow from the issue summary so could we update that with steps to reproduce and proposed solution.

This will also need a unique test. Not sure if editing that one counts as a valid test.

Thanks.

Re-roll patch #16 for 10.x

For #19