I solved my issue in the ResponsiveTableRecords RichWidget style sheet by replacing "before" pseudo-element with "after". Exemple below :
.Responsive .TableRecords td:first-child:after {
content: "\f0dd";
float: right;
font-family: FontAwesome;
color: #AAA;
margin-right:5px;
}
Now the arrow is displayed correctly when responsive is activated on small screens :

My feeling is that there is a conflict in CSS. Using "after" instead of "before" is a workaround and contributes to solve this conflict.
I hope that this could help anyone having such problem.