Sync property of Animation Nodes only works with single Animation Nodes. More complex branches are stopped.

### Tested versions

Reproducible in Godot 4.3 Dev 5

### System information

Ubuntu 22.04

### Issue description

The sync property of Animation Nodes are supposed to let both connected branches to go on even at extreme blend amount values. However this only works if such branchs are single animation nodes. This does not work if a branch is more complex, e.g., has Blend2 or other connected nodes.
In the video, we can see that the Blend2 node sync option is enabled. The first connected branch has another blend2 node (Blend2 2). We can see that this first branch is stopped when setting the blend amount to 1 although the sync option is enabled.

This is a problem when trying to synchronize different animation trees, e.g., synchronizing camera and player animation trees.

https://github.com/godotengine/godot/assets/8985409/19d0217a-1d21-4e35-8ba7-dd96d5ad2336

### Steps to reproduce

Download the attached project and play with the Blend2 node values to the extremes.

### Minimal reproduction project (MRP)

[syncanimationnode.zip](https://github.com/godotengine/godot/files/14681144/syncanimationnode.zip)

Start by running the attached syncanimationnode.zip project in Godot 4.3 and reproduce the Blend2 behavior with the sync option enabled at extreme blend values. Trace how nested Blend2 or other connected Animation Nodes handle synchronization; done means both complex connected branches continue instead of one stopping.

Written by the indexing model from the issue text.