I can see someone had an issue a while ago like this, but I’m on the latest version.
Nested flows do not seem to be working in this version. With or without arguments.
Try the following:
<script is="dmx-flow" id="flow2" type="text/dmx-flow">{
bootbox.alert: {title: "Hello!", message: "!!!"}
}</script>
<script is="dmx-flow" id="flow1" type="text/dmx-flow">{
subflow: {action: "{{flow2.run()}}"}
}</script>
<button id="btn2" class="btn btn-secondary" dmx-on:click="flow2.run()">Button</button>