The first time Ui Icon block is displayed, JS is not called

The demo bootstrap fixture is unstable because of missing dependencies, the icons are loaded from some specific icons packs. That's why you have the block 'icon' without the icon loaded.
To work properly it needs from https://gitlab.com/ui-icons/ui-icons-example: ui_icons_drupal, ui_icons_fontawesome (with lib, see readme), ui_icons_heroicons (with lib).

I disagree, ui_suite_bootstrap implements icons thanks to .icons.yml file, same as the icons libraries you mention.

I have the same issue with ui_icons_drupal.

Debugging, i can confirm the first time i display input an type text, no JS is loaded nor called.

See https://www.drupal.org/files/issues/2025-07-13/display-builer-icons-no-j... →

Can you reproduce without loading a fixture?
Can you reproduce if you create your own fixture with valid icon ids you have installed and working?