Annoying symbol % on windows when start tmuxp session

Step 1: Provide a summary of your problem

when i load my tmuxp session

tmuxp load .

it print annoying % symbol on each window

%                                                    
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           ~/projects/py-crawling-goodies git:(master) ✗

cat .tmuxp.yaml
session_name: pcg
start-directory: ./
shell_command_before: "conda activate pcg"
windows:
  - window_name: code
    focus: true
    panes:
      - shell_command:
        - nvim
  - window_name: cmd1
    panes:
      - shell_command:
        - sleep 1
        - clear
  - window_name: cmd2
    panes:
      - shell_command:
        - sleep 2
        - clear
  - window_name: mc
    panes:
      - shell_command:
        - sleep 3
        - clear

Step 2: Provide tmuxp details

python --version
Python 3.7.4

echo $PATH
/usr/local/Caskroom/miniconda/base/bin:/usr/local/Caskroom/miniconda/base/condabin:/usr/local/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/opt/fzf/bin

tmux -V
tmux next-3.2 (same with previous version 3.0)

tmuxp -V
tmuxp 1.5.4

shell: zsh

Step 3: Describe your environment

• Architecture: Darwin Kernel Version 19.4.0
• OS version: Catalina 10.15.4

Reproduce the issue with tmuxp load . and the provided .tmuxp.yaml on the documented Python, tmux, tmuxp, zsh, and macOS setup. No source file or test is identified in the report; trace session and window startup handling, and consider the issue done when loading the session no longer prints the unwanted % symbol.