That 'glitch' is markup from when the map was being originally generated. On the terrain version of the map, the line was red, as seen in TerrainMap below, circled in yellow. These markup lines are usually used to denote rivers but can also be used to trace any terrain feature the devs want to highlight. I've also seen other colors used on the really rough version of map tiles. Usually these marks get painted over when the actual terrain is generated, but sometimes when they are far enough out of the bounds that players can reach, the devs don't bother. In this case, a small area got left unpainted in the valley on the backside of a mountain ridge. I'm not sure why the line shows up in blue on the parchment map of Car Bronach, probably part of the process involved in converting the generated terrain map to an art map, but it is definitely the same line. It's just a glitch, there are lots of them. Mordor has many. They aren't really worth noting.
ScreenShot_2021-12-22_111855_0
FWIW, if there is any part of a parchment map where you would like to see the underlying terrain, just open the map in MoorMap, then left click on the location in question and select 'Show in TerrainMap' and TerrainMap will open centered on the location clicked with a flashing ping at the location (just click on the ping to make it disappear). This can make understanding the parchment maps easier - note, you have to have both MoorMap and TerrainMap installed to use this functionality.
And ouch. This exercise has made me realized that I missed a tile in Car Bronach (that little grey square in the top left of the screenshot)! Grrrrrrrrr. Not sure how that got past the QA goblins but somebody is in for a whipping...
EDIT: You can fix the missing tile by editing Grid.lua in the TerrainMap folder and adding the following line at the end:
Grid[2][8][70]={[1]=0x412105a8}
EDIT2:
In retrospect, it would seem that the line is not a glitch. It is a river. As I noted earlier, red lines on the markup are usually used to indicate rivers. The reason it shows up bluish-green on the map is that it is indicating water - note the greenish tint to the water near the Angmarim camp. It's just a river, granted it's in an odd location and seems to terminate abruptly, but still, just a river. No glitch, no bug, a total non-issue.