Array += Array is broken

[vmedea]

Godot version

4.1.dev (4dcf2c5)

System information

Debian Linux, rendering N/A

Issue description

I noticed the following strange behavior on master as of 4dcf2c5:

func _ready():
        var list: Array = []
        list = [1,2,3,4]
        list += [1,2]
        print(list) # should print: [1,2,3,4,1,2] prints: [1,2]
        list += []
        print(list) # should print: [1,2,3,4,1,2] prints: []

        var list2: Array = [1,2,3]
        list += list2
        print(list) # should print: [1,2,...2,3] prints: [1,2,3]

It appears that += overwrites the current data instead of appending.

FWIW, just writing var list = [] doesn't trigger the problem. So it's likely something with type Array. Array.append_array doesn't suffer from the problem.

The problem is similar to #72948, however this involves not a typed array only the type Array, and I haven't seen an error. The data just disappears silently. But it might still be fixed by #73540. Haven't tested yet.

Steps to reproduce

See issue description.

Minimal reproduction project

N/A

[AThousandShips]

Related to #75832, and is fixed by #75885