true issue, caused by wrong navmesh settings. if you move/copy the tree it creates a little part around it/them.
workaround:
select “RecastNavMesh Default” in World Outliner
and under “Generation” tab
press the little arrow to see advanced settings
and there check “Do Fully Async Nav Data Gathering”
moreover it helps you to evaluate navmesh issues if you check “Enable Drawing” under “Display” tab.