1. Version
   InDesign 16.4.1 CC2021 (macOS 10.16 Mojave)
   Only in applescript, not in javascript.

2. Steps to reproduce the issues
   step1. prepare a spreads document. Total number of pages must be an odd number.
   step2. Use AppleScript to set the new section at even-numbered page.

3. Expected result
   The state of the spread is not changed.

4. actual result
   The spread at the end is split into two spreads.

5. --code

main()

on main()
mySetup() of me

tell application "Adobe InDesign 2021"

    set mydoc to document 1
    set mypage2 to page 2 of mydoc

    tell mydoc
        set newsection to make section with properties {continue numbering:false, page start:mypage2, page number start:4}
    end tell
end tell

end main

on mySetup()
tell application "Adobe InDesign 2021"
set mydoc to make document
set properties of document preferences of mydoc to {facing pages:true, pages per document:5}
end tell
end mySetup

-- code end

In Japanese:

見開きドキュメントを準備。総ページ数は奇数。
任意の偶数ページにセクションを設定するスクリプトを実行すると、最終スプレッドが分割されてしまう。
Applescriptでのみ発生して、javascript や手作業では発生しない。