Hi,
on my Edge 830 (Software Version 5.50) some of the segments from my Strava list do not show up.
There are even segments that seem to be downloaded to the device, but appear as "empty" segments with a length of 0 meters:
(for this photo I deliberately cleared my segment list in Strava except for the segment in question in order to make the problem clear).
I could reproduce the same behavior on my older Edge 520.
Neither synchronizing the Edge with Garmin Express from Win10 instead with Garmin Connect (Android) nor reinstalling Garmin Connect helped so far.