Not all segments are displayed on the map at once. Many times I’ve seen segments appear and disappear when zooming in / out, with no clear pattern. 

If you post a link to the specific segment, I can try if I can see it on the map.

On the phone app, hidden segments won’t be visible really in any way other than clicking on the achievement.  On the desktop, did you scroll to the bottom of the list of segments and click on the button to view the hidden segments?  

I found the list of segments and the “Show Hidden Effort” button - there are so many nooks and crannies on this website.  And yes, apparently, it’s a hidden segment.  So, why doesn’t it show up here? 

That list is for segments that you have marked as hidden.  Essentially, there are the popular segments that are visible right up front when you look at your activity file. Less popular segments, ones that either aren’t ridden much or have been hidden but a lot of other users will not display initially unless you click the “view hidden segments” button at the bottom of the segment list.  This is just a way to avoid all of the clutter of bad and duplicate segments that people are tired of seeing all the time.  Those, either displayed or not, will not always correlate to segments that you have chosen to hide, thus the list you are referencing.  You can hide any segment you want so that it doesn’t show up on your list even though others may like them and want them to show on theirs.  This gives you a way to see which ones you have hidden yourself and gives you the option to “unhide” them if you decide now that you do actually want to see it normally.  

Hopefully that helps, and yes, there are a lot of little quirks with the functionality of the website and especially with the differences in how things work between the app and the web versions.  There are multiple other threads on here about those subjects….