martinus-Amember Having tried to replicate the same at my local install however, the specific icon isnt appearing even after adding the Css rule-set to render the pseudo element in custom.css file :
.ion-ios-link-outline::before{content: "\f1ca";display: inline-block;font-family: "Ionicons";speak: none;font-style: normal;font-weight: normal;font-variant: normal;text-transform: none;text-rendering: auto;line-height: 1;-webkit-font-smoothing: antialiased;-moz-osx-font-smoothing: grayscale;}
As apparently seen, It appears to be issue with their CSS content values not reflecting as envisaged. The issue can also be apparently seen with CSS content values for several other icons from similar library wherein other icons are showing instead of one showing in their demo site. ref: screenshot
Also, Kindly confirm if font-awesome icons are reflecting fine for you from varied selections ?
You may also contact the developer/support team of specific icons for the relative issue