Bubble uses a Google Webfonts library to preload fonts. For fonts loaded from a custom source (rather than Google Fonts), the library appends :::custom to the font name as part of its internal format. This suffix appears when you access the font_face property in plugin code.
What you might notice
Accessing the font_face property for a custom font in your plugin returns the font name with :::custom appended (e.g. "MyFont:::custom")
Google Fonts don't exhibit this behaviour — only fonts loaded from a custom source
Workaround
In your plugin code, strip the :::custom suffix from the font_face value when you need just the font name. You can read more about how the font_face property works in the plugin documentation.