Make sure you still return "\n" from your service. If it is the case, then the issue must be related to your mashup structure. Start checking it step by step, i.e. execute this in your DevTools console:
document.querySelector('ptcs-chart-bar')
document.querySelector('ptcs-chart-bar').shadowRoot.querySelector('ptcs-chart-axis')
document.querySelector('ptcs-chart-bar').shadowRoot.querySelector('ptcs-chart-axis').shadowRoot.querySelectorAll('ptcs-label')
document.querySelector('ptcs-chart-bar').shadowRoot.querySelector('ptcs-chart-axis').shadowRoot.querySelectorAll('ptcs-label').forEach(label => {
console.log(label);
});
Also, those selectors act on the first chart in the mashup only. If you have more than one chart, it may work unexpectedly. Basically, troubleshoot it step by step, comparing it to the DOM you see in DevTools > Inspect.