According to the Bootstrap docs...
"All table styles are inherited in Bootstrap 4, meaning any nested
tables will be styled in the same manner as the parent."
So in order to override the stripes on the nested table you'd have to add a little CSS...
.table-nostriped tbody tr:nth-of-type(odd) {
background-color:transparent;
}
<table class="table table-bordered table-sm table-striped">
<tbody>
<tr>
<td>
<table class="table table-sm table-borderless table-nostriped">
</table>
</td>
</tr>
</tbody>
</table>
https://www.codeply.com/go/95b4ecfFP7