[Bug]: Alternate prompts don't work correctly with attention emphasis

Is there an existing issue for this?

[X] I have searched the existing issues and checked the recent builds/commits

What happened?

The prompt [cat|dog] alternates the prompts per step. If you try and add attention / emphasis, the token gets ignored, so:

[(cat:1.1)|(dog:0.9)]

returns random garbage.

Steps to reproduce the problem

Go to txt2image tab.
Enter the prompt:

[(cat:1.1)|(dog:0.9)]

Random garbage is generated. This happens with 1.4 or 1.5 models.

What should have happened?

It should alternate the prompt and respect the weightings.

Commit where the problem happens

3e15f8e0f5cc87507f77546d92435670644dbd18

What platforms do you use to access UI ?

Windows

What browsers do you use to access the UI ?

Google Chrome

Command Line Arguments

--xformers --vae-path "C:\Users\<user>\Documents\GitHub\stable-diffusion-webui\models\Stable-diffusion\vae-ft-mse-840000-ema-pruned.vae.pt" --ckpt "C:\Users\<user>\Documents\GitHub\stable-diffusion-webui\models\Stable-diffusion\SD_1_5.ckpt"

Additional information, context and logs

No response

AUTOMATIC1111 / stable-diffusion-webui

[Bug]: Alternate prompts don't work correctly with attention emphasis #3686