The problem is that the FormLayout (AnchorLayout by extension) is not really built to handle sizing of subcontainers well (a tabpanel in your case). The way to handle this would be to use a vbox, put the top two fields in a container (simple div) and they can use a form layout as normal. Then flex the TabPanel below and this should fill the remaining space properly. Tested this under 3.1.1 and 3.2-rc.
Theis was closed because the form layout is doing what it was designed to do. The anchors you set are relative to the entire container, not to what is remaining in that container. Your anchor: '100% 100%' setting for the tabpanel means the height of that component fills 100% of the container's entire width and height, not 100% of what remains available at the time the anchor is set.
I'll get an item opened for the designer team to look at the flex for vbox, sounds like an issue there.