Dropbox icon is missing in Juno, and the Loki workarounds (Dropbox icon missing in Loki) doesn't work anymore.
Try this workaround from Matt Harris. I've tested with Skype and it works.
This thing was very annoying to me and I decided to create this simple native wing-panel indicator. It is not perfect but it works very well and I'm still improving, you can try it!
the "problem" is Wingpanel-indicator-ayatana.
is applicable to almost all wingpanel "problems"
This is because the creators of elementary os think bigger than steam, dropbox, skype, mega, "several famous applications commonly used by users", etc.
Have a nice day.