Это решает, что вы используете класс "sticky-top", который добавляет z-index к заголовку на 1020, поэтому всегда появляется следующий текст поверх выпадающего списка. Просмотрите мою скрипку ее удалите на третьих.
<!--
Bootstrap docs: https://getbootstrap.com/docs
-->
<div class="accordion" role="tablist">
<div class="card" ng-repeat="">
<div class="card-header" id="heading-1" style="top:0;" data-toggle="collapse" data-target="#myaccordion">
<div class="dropdown mx-3">
<div class="btn btn-default dropdown-toggle" data-toggle="dropdown">
mydropdown
</div>
<div class="dropdown-menu" style="min-width: 8rem;">
<a class="dropdown-item">
<div>1</div>
</a>
<a class="dropdown-item">
<div>2</div>
</a>
<a class="dropdown-item">
<div>3</div>
</a>
<a class="dropdown-item">
<div>4</div>
</a>
<a class="dropdown-item">
<div>5</div>
</a>
</div>
</div>
</div>
<div id="myaccordion" class="collapse" data-parent="#accordion" aria-labelledby="heading-1">
<div class="card-body" style="background-color:white;z-index:0;">
hello world
</div>
</div>
</div>
<div class="card" ng-repeat="">
<div class="card-header" id="heading-1" style="top:0;" data-toggle="collapse" data-target="#myaccordion1">
<div class="dropdown mx-3">
<div class="btn btn-default dropdown-toggle" data-toggle="dropdown">
mydropdown
</div>
<div class="dropdown-menu" style="min-width: 8rem;">
<a class="dropdown-item">
<div>1</div>
</a>
<a class="dropdown-item">
<div>2</div>
</a>
<a class="dropdown-item">
<div>3</div>
</a>
<a class="dropdown-item">
<div>4</div>
</a>
<a class="dropdown-item">
<div>5</div>
</a>
</div>
</div>
</div>
<div id="myaccordion1" class="collapse" data-parent="#accordion" aria-labelledby="heading-1">
<div class="card-body" style="background-color:white">
hello world
</div>
</div>
</div>
<div class="card" ng-repeat="">
<div class="card-header" id="heading-1" style="top:0;" data-toggle="collapse" data-target="#myaccordion2">
<div class="dropdown mx-3">
<div class="btn btn-default dropdown-toggle" data-toggle="dropdown">
mydropdown
</div>
<div class="dropdown-menu" style="min-width: 8rem;">
<a class="dropdown-item">
<div>1</div>
</a>
<a class="dropdown-item">
<div>2</div>
</a>
<a class="dropdown-item">
<div>3</div>
</a>
<a class="dropdown-item">
<div>4</div>
</a>
<a class="dropdown-item">
<div>5</div>
</a>
</div>
</div>
</div>
<div id="myaccordion2" class="collapse" data-parent="#accordion" aria-labelledby="heading-1">
<div class="card-body" style="background-color:white">
hello world
</div>
</div>
</div>
<div class="card" ng-repeat="">
<div class="card-header" id="heading-1" style="top:0;" data-toggle="collapse" data-target="#myaccordion3">
<div class="dropdown mx-3">
<div class="btn btn-default dropdown-toggle" data-toggle="dropdown">
mydropdown
</div>
<div class="dropdown-menu" style="min-width: 8rem;">
<a class="dropdown-item">
<div>1</div>
</a>
<a class="dropdown-item">
<div>2</div>
</a>
<a class="dropdown-item">
<div>3</div>
</a>
<a class="dropdown-item">
<div>4</div>
</a>
<a class="dropdown-item">
<div>5</div>
</a>
</div>
</div>
</div>
<div id="myaccordion3" class="collapse" data-parent="#accordion" aria-labelledby="heading-1">
<div class="card-body" style="background-color:white">
hello world
</div>
</div>
</div>
<div class="card" ng-repeat="">
<div class="card-header" id="heading-1" style="top:0;" data-toggle="collapse" data-target="#myaccordion4">
<div class="dropdown mx-3">
<div class="btn btn-default dropdown-toggle" data-toggle="dropdown">
mydropdown
</div>
<div class="dropdown-menu" style="min-width: 8rem;">
<a class="dropdown-item">
<div>1</div>
</a>
<a class="dropdown-item">
<div>2</div>
</a>
<a class="dropdown-item">
<div>3</div>
</a>
<a class="dropdown-item">
<div>4</div>
</a>
<a class="dropdown-item">
<div>5</div>
</a>
</div>
</div>
</div>
<div id="myaccordion4" class="collapse" data-parent="#accordion" aria-labelledby="heading-1">
<div class="card-body" style="background-color:white">
hello world
</div>
</div>
</div>
<div class="card" ng-repeat="">
<div class="card-header sticky-top" id="heading-1" style="top:0;" data-toggle="collapse" data-target="#myaccordion5">
<div class="dropdown mx-3">
<div class="btn btn-default dropdown-toggle" data-toggle="dropdown">
mydropdown
</div>
<div class="dropdown-menu" style="min-width: 8rem;">
<a class="dropdown-item">
<div>1</div>
</a>
<a class="dropdown-item">
<div>2</div>
</a>
<a class="dropdown-item">
<div>3</div>
</a>
<a class="dropdown-item">
<div>4</div>
</a>
<a class="dropdown-item">
<div>5</div>
</a>
</div>
</div>
</div>
<div id="myaccordion5" class="collapse" data-parent="#accordion" aria-labelledby="heading-1">
<div class="card-body" style="background-color:white">
hello world
</div>
</div>
</div>
</div>