It is shard to so say for sure what is happening without seeing the database. Nevertheless I suggest to use the other two shapes representing other two voltages as a template.
You already assigned that layer to be Split/Mixed plane. Follow below steps:
1. Change to layer in question to be an active layer
2. Decide what is the voltage that will be the dominant one
3. Select Board Outline as a shape
4. RMB (right mouse button) and select 'Create Plane Area'. This will create a plane area mimicking the board Outline
5. From next dialog select a Net for this plane shape it will belong to
6. From Drafting Toolbar select icon 'Plane Area' and start creating inside the plane are you just created new plane shape using older plane as a template. It will automatically auto separate from another plane. Assign Net the the new Plane area.
7. Repeat step 6 process for the third plane are.
8. Delete older shapes representing two older nets
If you still having a problem, contact Customer Support
Took a couple tries but it seems to have worked.
I will save it and reopen it to see if it stays correct.