Applescript `document` method not recognized

Steps to reproduce

1. Create an Applescript that opens MacVim and calls document 1. Eg:

tell application MacVim
    activate
    set my_path to get path of document 1
end tell

2. Execute script with osascript script_name.
3. Find the error: Can’t get document 1. Invalid index. (-1719)

See more details here.

Expected behaviour

get path of document 1 should return the file path of the first document being currently displayed. I believe it's also called the 'front document'.

Version of Vim and architecture

9.0.65 (173)

Environment

Operating System: Bug Sur version 11.6.8 (20G730)
Terminal: Using MacVim GUI, not a terminal. It's version shows the Vim version 9.0.65 (173). It also says c2007 Bjorn Winckler.
Value of $TERM: xterm-256color

How MacVim was installed

brew install macvim

Logs and stack traces

Don't find any log or other error besides what's displayed.  Maybe I don't know how to turn on verbose logging?

Vim configuration where issue is reproducable

No response

Issue has been tested with given configuration

• by running MacVim.app from GUI macOS interface
• by running vim/gvim/etc installed by MacVim
• by running other versions of vim (e.g. /usr/bin/vim)

Issue has been tested with no configuration

• by running mvim --clean (or gvim, supplied by MacVim distribution)
• by running vim --clean (in terminal, supplied by MacVim distribution)
• by running vim --clean (in terminal, other suppliers, e.g. /usr/bin/vim)

Other conditions

• The both Homebrew packages "vim" and "macvim" are installed

No source file or test is identified. Start by reproducing the failure with the shown AppleScript and inspect MacVim's AppleScript dictionary and document handling; done means get path of document 1 returns the path of the front document without the invalid-index error.