Wt

When you apply Wt::WBootstrap5Theme(), Wt hardcodes the legacy Bootstrap 3 class "in" to animate tooltips when you hover over them.

Modern Bootstrap 5 styling expects the class "show". Because the tooltip has "in" instead of "show" some Bootstrap 5 libraries explicitly hide any .fade element that doesn't have the .show class.

Wt injects

std::string WBootstrap5Theme::utilityCssClass(int utilityCssClassRole) const
{
  switch (utilityCssClassRole) {
  case ToolTipInner:
    return "tooltip-inner";
  case ToolTipOuter:
    return "tooltip fade top in position-absolute";
  default:
    return "";
  }
}