I had the same problem.
Here is the CSS code which fixed the problem for me.
You need to set the container to position absolute.
@import "@arcgis/core/assets/esri/themes/light/main";
:host {
width: 100%;
height: 100%;
}
.map-wrapper {
position: relative;
height: 100%;
width: 100%;
.map-content {
position: absolute;
height: 100%;
width: 100%;
margin: 0;
padding: 0;
}
}
<div class="map-wrapper">
<arcgis-map class="map-content" />
</div>