Unofficial Skyrim Special Edition Patch

https://www.afkmods.com/index.php?/topic/10531-ussep-navmesh-errors/

Solitude navmesh 00037F7B was copied into US*P when other vertices were raised to ground level. The problem was around some rocks.

*** Not Found Triangle 31 in [NAVM:00037F7B] (in GRUP Cell Temporary Children of [CELL:00037EE3] (in SolitudeWorld "Solitude" [WRLD:00037EDF] at -14,24))
*** Not Found Triangle 44 in [NAVM:00037F7B] (in GRUP Cell Temporary Children of [CELL:00037EE3] (in SolitudeWorld "Solitude" [WRLD:00037EDF] at -14,24))
*** Not Found Triangle 45 in [NAVM:00037F7B] (in GRUP Cell Temporary Children of [CELL:00037EE3] (in SolitudeWorld "Solitude" [WRLD:00037EDF] at -14,24))
*** Not Found Triangle 46 in [NAVM:00037F7B] (in GRUP Cell Temporary Children of [CELL:00037EE3] (in SolitudeWorld "Solitude" [WRLD:00037EDF] at -14,24))

Turns out, the real problem was Triangle 40 has a downfacing normal.

There was a 1 triangle island 000ECFFC whose vertices were hidden by this 00037F7B vertices, likely vanilla autogenerated. This causes an invisible hole beside triangle 40.

An easy solution was to move the island on top of a nearby barrel, add edge jumps to it, add a new triangle to fill the hole, and adjust the other nearby vertices for fewer long skinny triangles.

Upon save, all these triangles now have the "found" flag. That means NPCs can properly walk through the area.