Reproduction steps:
1. Open the attached "Archive" project.
2. Check three scripts in the "Assets" folder.
- Each one takes different argument type(int, float, long).
- Each one is attached to a different game object.
- Each one is called by a different button On Click function.
3. Expand "Canvas" in the hierarchy.
4. Select "button" in the hierarchy.
5. Select "Button (Script) > OnClick () > script selection > Script" in the hierarchy (Picture attached).
- Note that test (int) is available.
6. Repeat steps 4 and 5 for "button 1".
7. Repeat steps 4 and 5 for "button 2".
- Note that test (long) doesn't show up in the function selection screen.

Reproduced with:
5.3.6p7,
5.4.2f1,
5.5.0b7