Please use Diagram’s client side updateLabel method to resolve your reported issue which allows the user to update the label at runtime. We have modified your JSPlayground link and provided below.
Please refer to the below KB link for more details
In your JSPlayground link, the label fontSize is working for the flowShape alone and not works for the BPMNShape. This is due to you have set the currentNode.labels.fontSize = size; for the flow shape and BPMN shape. So the flow shapes is a single node and the labels changes is reflected directly while updating node but the BPMN shape is a group node and has more children. So the labels changes is not reflected directly while updating the node. you need to use our diagram’s client side updateLabel method to update the labels at runtime.