Drupal core provides a form-text class to some fields (i. e. text fields). form-text class also has a special meaning in Bootstrap. So this collides in some points and Bootstrap5 theme is setting $form-text-margin-top variable to fix this issue I suppose.
But this does also affect the "regular" use of the Bootstrap form-text class, and you may not be able to easily override the variable yourself.
I would propose to not set the Bootstrap variable, but instead fix the issue in the Drupal styles, by setting a zero margin to input.form-text selector. This fixes the issue with the form-text class from Drupal, but still let us make use of the Bootstrap form-text class if we want to and override the Bootstrap variable in case.
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4.0
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thank you for your contribution.
tested and ready to be merged in.