TYPO3 Core

DCE version: 2.0.6
TYPO3 version: 9.5.8 LTS
PHP version: 7.2 =========================

When adding a checkbox to a section and trying to tick (check) it in the backend will only have effect to the very first element of the section array.

First of all I thought the checkbox was broken or had a wrong config (all elements but the fifth were collapsed, I tried to check the fifth box -> the checkbox did not show any reaction, but acting like "readonly")
After having a look to the configuration and not finding any error (by the way ... how should I? A checkbox only have 2 lines of config :D ) I expanded all section fields and recognized, that whatever I tick, only the first element will show effect.

I assume a missing dependency between checkbox and section Nth element.