not working with field group

I assume your signature field is in a group that's hidden by default? Apparently this is a limitation of the element, it cannot be "initialized" in a hidden element.

To fix this I would have to adjust the initialization to detect if the widget is visible and if not postpone further processing until it becomes visible. That sounds like a lot of work/changes for a relatively small added value. I'm not gonna invest time in this, but feel free to open an MR and I'll review your changes.