When trying to select 'Random Between Two Constants' for SubEmitter parameters like Start Lifetime, Start Speed etc., the parameter remains in 'Constant' state.

Steps to reproduce:
1) Open the attached project.
2) Select 'SubEmitter0' in the Hierarchy.
3) In the Inspector, go to SubEmitter0 > Start Speed and click the small, down-pointing arrow on the right next to the entry.
4) Select 'Random Between Two Constants'.

Two value boxes should appear, however only a single box remains, and the parameter is still set to 'Constant'. The same occurs for 'Random Between Two Curves', but the curve graph appears in the bottom of the Inspector. This does not occur for the parent particle system.

Reproduced on:
5.6.0b1, 5.6.0a6

Did not reproduce:
5.5.0p1, 5.4.3p3

Reproduced on macOS 10.12.1 and Windows 10.