Clearance between planes in a split plane layer?
Can anyone advise what the minimum distance should be between planes on an internal split plane layer? My designs are usually 5V mixed signal circuits.


As far as manufacturability goes, the minimum is usually the same as the minimum spacing on any other layer. 4 mil or 0.1 mm is a reasonable choice if you haven't chosen a vendor yet and are using 1/2 or 1 oz copper weight. If you are using heavier copper, you may need to increase the spacing --- contact your vendor for details.


  My designs are usually 5V mixed signal circuits.


If the analog power supplies are exceptionally sensitive (for example, you're doing 20-bit or more conversions) then increase the gap between analog supply voltages and other nets. How much will depend on the exact details of your design. 100 mils (2.5 mm) is most likely sufficient for all but the most sensitive circuits.


  Can anyone advise if this is expected behavior of the CAD package, or am I somehow missing some feature that I need to enable or turn on?


This is expected. What you draw is what you are telling the tool you want to build. 

However if you gave a 10 mil design rule and used a 4 mil line to separate planes, you should get an error when you run batch DRC.