mdbTableSort is not working

I generate a Table-Header like this:

            <thead class="sticky-top">
            <tr>
                <th *ngFor="let head of headElements; let i = index" [mdbTableSort]="elements" [sortBy]="head.sort" 
                    scope="col" [style.width]="(head.col == 'bem' ? '200em' : '60px')">{{head.head | titlecase}} <mdb-icon fas icon="sort"></mdb-icon>
                </th>
            </tr>
        </thead>

and here the Table-Body:

        <tbody #row>
            <tr mdbTableCol *ngFor="let el of elements; trackBy: trackByFn; let i = index">
                <td>
                    {{el.adrNr}}</td>
                <td>
                    {{el.name}}</td>
                <td>
                    {{el.reNr}}</td>
                <td>
                    {{el.status}}</td>
                <td>
                    <span>{{el.reDat | localizedDate:'dd.MM.yyyy'}}</td>
                <td>
                    {{el.fallDat | localizedDate:'dd.MM.yyyy'}}</td>
                <td>
                    {{el.obj}}</td>
                <td>
                    {{el.mahnSperre}}</td>
                <td>
                    {{el.offBet | number : '.2-2'}}</td>
                <td>
                    <textarea type="text" id="form7"
                                        class="md-textarea md-textarea-auto form-control" mdbInput
                                        style="font-size: 11pt"
                                        [ngModel]="el.bem"
                                        maxlength="600"
                                        (ngModelChange)="saveComment(el.dto, $event)"></textarea>
                    </td>
            </tr>
        </tbody>

The sorting is generally working perfect. But one customer uses a Browser-Extension which adds a self implemented span (tcxspan) to the data:

<td _ngcontent-lrc-c653="">
   <span _ngcontent-lrc-c653="">
       <tcxspan tcxhref="2173706" title="Call 2173706 via 3CX">2173706</tcxspan>
    </span>
</td>

Now, the sorting is not working anymore. Can someone help me what I can do here?

Thank you.

Best Regards THomas