Editing StringName argument adds extra &

[KoBeWi]

Tested versions

4.3 dev6

System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 31.0.15.4633) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

Issue description

From #66481

godot.windows.editor.dev.x86_64_P3hgkOWLyg.mp4

Notice how quotes are correct, but the prefix is not.

Steps to reproduce

1. Write some method with StringName argument and completion (e.g. is_action_pressed()) and use some argument
2. Move caret inside the string and autocomplete for different argument

Minimal reproduction project (MRP)

N/A