Repro steps:

1. Add a "Slider" component to the scene.
2. Select it and check its "On Value Changed" event property.
3. See that it has no argument and no method that takes a float can be assigned to it.

This has been found in 2019.3.0b1. The problem is not present in 2019.1. 2019.2 has not been checked.

The problem seems to appear for all UnityEvents that take arguments. If it does, the argument does not appear in the inspector, any existing method on it will appear as "missing", and no new method with the right signature (e.g. expecting a float) can be assigned.

Note that this is not exclusive to uGUI. Any use of UnityEvent anywhere will exhibit the same problem.