When I open the Timeline field and tick the “Set as milestone” it shows the icon, but as soon as I click away from the field it disappears, its happening on all of our boards by all users.
This appears to be a small bug. If you haven’t already, I would suggest trying the following:
Refresh your browser and try to set the milestone again
Try an incognito window
Try a different supported browser (Safari, Chrome, Firefox)
Just for clarification also, when setting a milestone date it is a singular date in the timeline. If you would still like a range, that would be the default settings in the timeline.