When you add two tabs to a tab pane with both of them containing split panes, then the divider position setting of the split pane in the second tab is ignored - it seems that the divider always appear at position 0.5 no matter what.
This issue appears somewhat similar to other issues that have been closed (like RT-21854).
Constructing a similar ui with the Builder seems to work, but is unfortunately not an option.
(I will try to attach a Netbeans project that demonstrates this issue, but I don't see an option to attach anything, maybe on another page...)